Traffic Engineering And Managementdocshare04.docshare.tips/files/28257/282573784.pdf · 2017. 2. 17. · traffic management systematically and scientifically with the use of concept

Traffic Engineering and Management /[email protected]

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Traffic Engineering Management

(BEG469TE)

(Elective II)

Year: 4 Semester

Teaching Schedule Hours/week

Examination Scheme Total marks Remarks

Final Internal Assessment

L T P Theory Practical Theory Practical

3 2 0 80 - 20 - 100

Course objectives:

The main objective of the course "Traffic Engineering Management" is to impart knowledge about

traffic management systematically and scientifically with the use of concept of engineering. Traffic

management as a burning issue and is of high importance for the developing cities, it should be

followed by the future traffic load analysis. Key topics of the course attempt to impart knowledge in

the following contemporary concepts:

Conceptual knowledge in traffic management system;

Issues, relative importance and methods of Transport Management;

This course may be good platform for the Graduate (Masters' degree) course in Traffic Engineering and

Management.

Course Contents:

1. Introduction 2 hrs.

1.1 Scope and significance of Traffic Engineering Management

1.2 Traffic planning and modeling using prototype

1.3 Traffic related problems in major cities

1.4 Transportation network and their characteristics

2. Urban Traffic Planning 3 hrs.

2.1 Introduction to urban traffic planning

2.2 Calculation of traffic volume

2.3 Travel demand forecasting

3 .Traffic Characteristics 3 hrs .

3.1 Basic traffic characteristics - Speed, volume and concentration.

3.2Relationship between Flow, Speed and Concentration

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4. Traffic Measurement And Analysis: 5 hrs.

4.1Volume Studies - Objectives, Methods;

4.2 Speed studies - Objectives: Definition of Spot Speed, time mean speed and space mean

speed;

4.3Methods of conducting speed studies;

5. Speed Studies: 5 hrs.


5.2Presentation of speed study data;

5.3Head ways and Gaps;

5.4Critical Gap;

5.5 Gap acceptance studies.

6. Highway Capacity And Level Of Service: 5 hrs.

6.1Basic definitions related to capacity

6.2 Level of service concept

6.3 Factors affecting capacity and level of service

6.4 Computation of capacity and level of service for two lane highways Multilane

highways and free ways.

7. Parking Studies And Analysis : 5 hrs.

7.1Types of parking facilities - on street parking and off street Parking facilities;

7.2Parking studies and analysis.

8 Traffic Safety: 7 hrs.

8.1Accident studies and analysis;

8.2Causes of accidents - The Road, The vehicle, The road user and the Environment;

8.3Engineering, Enforcement and Education measures for the prevention of accidents.

9 Traffic Control And Regulation: 5 hrs.

9.1Traffic Signals - Design of Isolated Traffic Signal by Webster method,

9.2Warrants for signalisation, Signal Co-ordination methods, Simultaneous, Alternate,

Simple progressic and Flexible progression Systems.

10. Traffic And Environment: 3 hrs.

10.1 Detrimental effects of Traffic on Environment;

10.2 Air pollution; Noise Pollution;

10.3 Measures to curtail environmental degradation due to traffic.

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11. Traffic Management in Nepal 2 hrs.

11.1 Overview of existing system and future trend

11.2 National Transport Policy, Five Year Plans

11.3 Existing planning process

Tutorials:

1. A case study on traffic measurement and analysis

References

1. Traffic Engineering and Transportation Planning - L.R. Kadiyali, Khanna Publishers.

2. Traffic Engineering - Theory & Practice - Louis J. Pignataro, Prentice Hall Publication.

3. Principles of Highways Engineering and Traffic Analysis - Fred Mannering & Walter

P. Kilareski, John Wiley & 50ns Publication.

4. Transportation Engineering - An introduction - C. Jotin Khistry, Prentice Hall

Publication.

5. Fundamentals of Transportation Engineering - C.S.Papacostas, Prentice Hall India.

Question Pattern:

Chapter Marks allocated Remarks

1 4

2 4

3 4

4 10

5 10

6 10

7 10

8 10

9 10

10 4

11 4

Total 80

***Above mentioned marks can be with minor variations.

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Contents CHAPTER ONE: INTRODUCTION ................................................................................................... 1

1.1 Scope and significance of Traffic Engineering and Management .................................................. 1

1.2 Traffic planning and modeling using prototype............................................................................ 2

1.3 Traffic related problems in major cities ....................................................................................... 3

1.4 Transportation network and their characteristics .......................................................................... 6

CHAPTER TWO: URBAN TRAFFIC PLANNING ............................................................................. 8

2.1 Introduction to urban traffic planning.......................................................................................... 8

2.2 Calculation of traffic volume.................................................................................................... 10

CHAPTER THREE: TRAFFIC CHARACTERISTICS ....................................................................... 16

3.1 Basic traffic characteristics - Speed, volume and concentration................................................... 16

3.2Relationship between Flow, Speed and Concentration ................................................................ 17

CHAPTER FOUR: TRAFFIC MEASUREMENT AND ANALYSIS .................................................. 21

4.1 Volume Studies ....................................................................................................................... 21

4.2 Speed studies .......................................................................................................................... 26

4.3Methods of conducting speed studies; ........................................................................................ 27

CHAPTER FIVE: SPEED STUDIES................................................................................................. 32

5.1Head ways and Gaps ................................................................................................................ 32

5.2 Uncontrolled Intersection ......................................................................................................... 33

5.3 Gap acceptance studies ............................................................................................................ 37

CHAPTER SIX: HIGHWAY CAPACITY AND LEVEL OF SERVICE .............................................. 45

6.1Basic definitions related to capacity........................................................................................... 45

6.2 Factors affecting capacity and LOS .......................................................................................... 47

6.3 Level of Service concept .......................................................................................................... 49

6.3 Computation of capacity and level of service for two lane highways, multilane highways and

freeways....................................................................................................................................... 50

CHAPTER SEVEN: PARKING STUDIES AND ANALYSIS ............................................................ 70

7.1Types of parking facilities - on street parking and off street Parking facilities ............................... 70

7.2Parking studies and analysis ...................................................................................................... 73

CHAPTER EIGHT: TRAFFIC SAFETY ........................................................................................... 78

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8.1Accident studies and analysis .................................................................................................... 78

8.2Causes of accidents - The Road, The vehicle, the road user and the Environment ......................... 80

8.3 Accident studies, records and analysis ...................................................................................... 81

8.3Engineering, Enforcement and Education measures for the prevention of accidents. ..................... 86

CHAPTER NINE: TRAFFIC CONTROL AND REGULATION......................................................... 90

9.1Warrants for traffic control signal system................................................................................... 90

9.2 Design Principles of Traffic Signal ........................................................................................... 90

9.3 Signal Co-ordination methods, Simultaneous, Alternate, Simple progression and Flexible

progression Systems...................................................................................................................... 99

CHAPTER TEN: TRAFFIC AND ENVIRONMENT ........................................................................106

10.1 Detrimental effects of Traffic on Environment........................................................................106

10.2 Air pollution; Noise Pollution................................................................................................109

10.3 Measures to curtail environmental degradation due to traffic ...................................................112

REFERENCES................................................................................................................................113

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CHAPTER ONE: INTRODUCTION

1.1 Scope and significance of Traffic Engineering and Management

Traffic engineering is one of the specialized areas of transportation engineering which is itself a branch of

civil engineering. It deals with traffic studies, analysis and engineering application for the improvement of

traffic performance on road. Institute of Traffic Engineers (USA): ―Traffic Engineering is that phase of

engineering which deals with planning, geometric design and traffic operations of streets and highways,

their networks, terminals, abutting lands, relationship with other modes of transportation for the

achievement of safe, efficient and convenient movement of persons and goods‖.

Necessity:

It is relatively new branch of civil engineering.

It became necessary with the increase in traffic (number of vehicles).

Traffic congestion, parking problem, environmental degradation, traffic accidents, has created the

attention to the performance characteristics of highway transportation and continuous study and

developments for better geometric design, capacity, intersections, traffic regulations, signals,

signs, roadway marking, terminals, street lighting etc.

Has been recognized as an essential tool in the improvement of traffic operation

Objective of the Traffic Engineering

Basic objective is to achieve efficient, free, and rapid flow of traffic with minimum number of traffic

accidents. Traffic engineering includes a variety of engineering and management skills and the followings

are the main aspects:

Traffic characteristics—vehicles and road users

Traffic study and analysis—speed, volume, capacity, traffic pattern, OD,

Traffic flow characteristics, parking and accident studies

Traffic operation, control and regulation—laws and traffic regulatory

Measures, installation of traffic control devices—signs, signals and islands

Planning and analysis—separate phase for expressways, arterial roads,

Mass transit facilities, parking facilities etc.

Designs—geometric design, parking facilities, intersections, terminals, lighting

Traffic administration and management—engineering, education and enforcement

Continual research

ROAD TRAFFIC MANAGEMENT: As urban populations expand and city roads become increasingly

congested, policy makers and planners need to review urban development and transport policies in order

to address future needs taking into account anticipated social and demographic changes.

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Effective policy must meet multiple objectives:

Strike a balance between different modes of transport: pedestrians, bicycles, motorcycles, cars

and public transport

Provide security, safety and optimum service for transport system users

Maintain the mobility that drives economic development

Reduce urban pollution and congestion caused by motor vehicles

Alongside longer-term solutions such as upgrading public transport systems and introducing city center,

road toll systems, high-performance traffic management systems can be crucial to the success of a city

planning and transportation policy

Traffic Management Solutions include:

Improved road user safety: better traffic control for improved road safety and shorter response

times by emergency services

Quicker travel times in urban areas: smoother traffic flows and shorter public transport journey

times

Less pollution: lower fuel consumption and less environmental impact

Widespread availability of road user information: accurate, reliable user information to

improve the travel experience

1.2 Traffic planning and model ing using prototype

As the number of traffic is increasing exponentially, traffic related problems has born. For the smooth and

effective traffic flow with minimizing traffic accidents and travel cost and maximizing the comfort and

easiness, traffic planning among the city has become inevitable. For the traffic planning, modeling using

prototype study is the best solution for the selection of best among the best alternatives.

Model concept

A model can be defined as a simplified representation of a part of real world-the system of interest-which

concentrates on certain elements considered for its analysis from a particular point of view. For the

analysis, any model made should be calibrated and validated to ascertain the realistic resemblance and the

same validated model is used for the further analysis.

Model calibration is the process by which the numerical values of the parameters of an assumed model

are determined. It is accomplished through the use of Statistical methods and based on experimental

knowledge that is observations, of the dependent and independent variables. These observations are

employed to estimate the numerical values of the model parameters that render the postulated model

capable of reproducing the experimental data. Several statistical goodness-of-fit tests, the one that best

describes the experimental data can then be selected. In this manner it is ensured that the selected model is

realistic. The term calibration refers to procedures that are used to adjust the values of a model's

parameters to make them consistent with observations.

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Model validation refers to the testing of a calibrated model using empirical data than those used to

estimate the model in the first place. It means to predict a situation from the past and to compare this with

the actual situation in the present (back casting). This is how scientific theories are tested, modified, or

replaced.

Figure 1Methodological steps of model building process

1.3 Traffic related problems in major cities

Exponential growing of number of traffic within the limited fixed facilities like highway, interchange,

bridge etc is itself a great traffic related problem in the major city like Kathmandu. Followings are the

major traffic related problems:

Road space and Traffic Congestion: According to reports there are 180,000 (most of them being two

and three wheelers) vehicles registered at The Bagmati Transport Office at present. Considering the

narrow roads and the small area that the city is built in, these vehicles are too many for a city like

Kathmandu. The prevailing high degree of congestion, despite relatively low number of vehicles (private

car ownership rate is relatively low though there is relatively high number of vehicles registered, the most

of them being the two and three wheelers) is often attributed to the small proportion of urban space

devoted to roads. It is also revealed that the annoying causes of traffic jams in the streets of the city are

due to large number of motorbike riders. Traffic congestion is already an important constraint to urban

productivity and the vehicular air pollution is increasing and posing a serious health threat to urban

population.

Accidents: There has been an unprecedented trend in traffic accident in Kathmandu valley. While the

vehicles are increasing in geometric proportion, the roads are being constructed at a snail's pace.

Accidents are increasing in number and severity. Accidents occur more during working days when the

traffic is heavy. According to a report by the Traffic Engineering and Safety Unit at the Departments of

Roads, the frequency of accidents is at the peak at 4 pm followed by 8 am. Pedestrians are the ones who

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are most at risk, followed by motorcycle riders. Accidents also occur when holidays are near and mostly

youngsters tend to drive under the influence of alcohol. Most accidents in the valley happen at

intersections. The places in Kathmandu that witness accidents frequently include Teenkune, Koteshwor,

Harihar Bhawan, Putali Sadak, Ring road and many other intersections, while nocturnal mishaps are more

frequent on the Ring Road, Kantipath and Naya Baneshwor because of over speeding.

The traffic condition: There is no doubt that the wide variety of traffic sharing the limited right of way is

a serious factor in congestion. Most road sections in Kathmandu city are not channelized for motor

vehicles, bicycles and pedestrians. The greater the pressure on road space, the more speeds of the slowest

moving vehicles tend to be reduced, and the potential of faster public, commercial and private vehicles are

wasted. Often pedestrians, market and parking activities intrude even the road space of major arteries. The

greater number of traffic accidents and lower overall average speed of the vehicles in the streets are

attributed to the large number of motorbikes and tempos.

Parking: It is one of the city's chronic problems, particularly in the Business Districts and other sites

where jobs and retail activities are concentrated. The limited road space is further reduced due to

encroachment of the road space by street shops, vehicles and bicycle parking. In particular, parking on the

sidewalks of the streets causes danger to pedestrians. In many cases, construction materials can be seen

placed at footpath and sometimes even on the roads thus forcing the pedestrians to walk on the roadway

which is primarily meant for the motor vehicles. This may cause a great deal of danger for the safety of

the pedestrians. Many buses have to be parked on the streets. Bus terminals have not been well planned

and cause a lot of transfer difficulties for the passengers.

Public transport: Public transport in Kathmandu city can be seen in general as a well-connected but

inadequate capacity is reflected in extreme overcrowding during long periods of peak hour traffic and it

takes a long time in reaching their destination. The development of public transport is often hindered by a

lack of capacity, low operating speed, and outdated equipment and management practices. As there is no

single bus terminus, finding the different places from where buses leave can sometimes be an experience

because there is a lack of information at public places. Also the seating arrangements in most of these

buses are such that you would hardly get to see the scene outside as you journey.

Pedestrians and cyclists: There is problem of movement by pedestrians and cyclists. Pedestrians (and

particularly the safety of pedestrians) are generally not accorded adequate priority by the city officials

responsible for planning and managing roads, as footpaths are inadequate and badly maintained.

Pedestrian crossings are placed in a long walking distance and many people simply don't cross the roads

using the overhead bridge. There are no any rules and regulations regarding punishment for those who

cross the road randomly. As a result walking and crossing streets in many places have become highly

dangerous. Conditions for cyclists are even worse than for pedestrians. Bicycle riding is increasingly

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hazardous. As a result, this cheap and potentially very important mode of transport tends to be grossly

underutilized.

Road maintenance: Roads are inadequately maintained. Visual inspection and evaluation of road

network conditions show failures of the road pavement. A key factor contributing to this situation is the

lack of funding for the maintenance by the government. The situation is exacerbated by the absence of

computer based asset inventory and maintenance management systems. The available scarce resources are

allocated to meet the most pressing demands. In addition to managing existing roads more efficiently

additional capacity is needed by the construction of new roads.

Urban patterns: Physical patterns of cities also compound the difficulties. Central business districts are

typically not so clearly demarcated as in the developed world. The main activities centers are however

often concentrated in narrow streets prone to the intense congestion. High densities of intersections,

winding configurations and changing road widths reduce capacity further.

Road user education: It has not been very efficient and had lacked proper methodology and facilities.

The striking feature of the city traffic is the poor driving behavior. Driving standards are generally low. It

may be amazing to know that many of the drivers have no idea about the traffic signs and rules, which

indicates that our license issuing system is also extremely unscientific and impractical, and it is helping in

adding traffic accidents indirectly. It is reported that in Kathmandu valley the number of accidents are

higher than in the rest parts of Nepal and it can be said that the root cause of increasing traffic accidents is

the lack of traffic awareness among drivers and also pedestrians.

Traffic control measures: Effective road capacity of the city is further reduced by extensive uncontrolled

parking of vehicles of all kinds and by ineffective signaling and other traffic control measures. Manual

control of junctions at peak hours is often required-land traffic signal timings are not appropriate. None of

all the existing traffic signals in the urban area are coordinated, most of them operating under two phase

fixed time control. Although there have been some successful experiments with junction channelization

recently in the city, the majority of the junctions have not been channelized and sometimes traffic island

itself is creating the traffic problem due to its inappropriate placement and bad design. Traffic signs and

markings are too much insufficient. Although some innovative pedestrian crossing facilities have been

implemented in the city, there is still a striking need for better provision of pedestrian crossing facilities to

give pedestrians safer ways to cross the road.

Remedial measures:

As mentioned earlier, with the very rapid growth in demand for transport, Kathmandu is facing serious

traffic problems. The immediate concern in the city is to maintain the existing levels of service of the road

system and personal mobility, whilst reducing the potential for road accidents. For this, traffic

management measures are to be utilized which typically will include junction improvements, one way

streets, segregation of two wheel vehicles with motor vehicle, channelization, markings, signaling,

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selective road widening and provision of pedestrian facilities, continuous traffic awareness program

through the involvement of all the sectors of the society. But traffic management is the concern of the

number of policy and executive agencies. As a result there is pressing need for close coordination,

effective decision making machinery and enforcement, and clearly defined responsibilities because the

success or failure of traffic management measures largely depend on the institutional arrangements.

1.4 Transportation network and their character istics

The transport system is represented by a network comprising of nodes and links that connect the nodes.

The network model is a simplified reproduction of the real network. The network is used to calculate the

travel times between points of origin and points of destination. The links of the network represent the

roads. The nodes of the network are the intersections. Nodes in the model network are also used to mark

changes in road types and the sites, for example of bridge and other specific infra – structure facilities.

The link may be:

Freeways: These roads provide largely uninterrupted

travel, often using partial or full access control, and

are designed for high speeds. Often freeways are

included in the next category, arterials. Arterials:

Arterials are major through roads that are expected

to carry large volumes of traffic. Arterials are often

divided into major and minor arterials, and rural and

urban arterials.

Collectors: Collectors (not to be confused with collector/distributor roads, which reduce weaving on

freeways), collect traffic from local roads, and distribute it to arterials. Traffic using a collector is usually

going to or coming from somewhere nearby.

Local roads: These roads have the lowest speed limit, and carry low volumes of traffic. In some areas,

these roads may be unpaved.

Link properties

Length

Travel speed

Capacity of link

Additional information about the link may be given

Type of road

Road width

Presence of bus lane, prohibition for certain vehicle etc

Banned turns

Type of junction Storage capacity for queues

Figure 2 Road Networks

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Approximate equivalence with road classification in other countries is as follows: class I roads correspond

to expressways, class II –to arterial roads, class III-to collector roads and class IV-to local roads.

In Nepal the overall management of National Highways and Feeder Roads comes within the responsibility of the

Department of Roads (DOR). These roads are collectively called Strategic Roads Network (SRN) roads. District

Roads and Urban Roads are managed by Department of Local Infrastructure Development and Agricultural Roads

(DOLIDAR). These roads are collectively called Local Roads Network (LRN) roads.

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CHAPTER TWO: URBAN TRAFFIC PLANNING

2.1 Introduction to urban traffic planning

As the traffic on the existing road system in cities grows, congestion becomes a serious. Medium and long

term solution like widening roads, providing elevated fly-overs and constructing bypasses and urban

expressways are costly. Simple and inexpensive solutions can tide over the crisis for some time. Planning

and managing the urban traffic could be a package of short term measures to make the most productive

and cost-effective use of existing transportation facilities, services.

The fundamental approach in traffic management measures is to retain as much as possible existing

pattern of streets but to alter the pattern of traffic movement on these, so that the most efficient use is

made of the system. In doing so, minor alternations to traffic lanes, islands, curbs etc. are inevitable, and

are part of the management measures. The general aim is to reorient the traffic pattern on the existing

streets so that the conflict between vehicles and pedestrians is reduced.

Some of the well-known traffic management measures are:

Restrictions on turning movements

One-way streets

Tidal-flow operations

Exclusive Bus-lanes

Closing side-streets.

Restrictions on turning movements

At a junction, the turning traffic includes left-turners and right-turners. Left-turning traffic does not

usually obstruct traffic flows through the junctions, but right-turning traffic can cause serious loss of

capacity. At times, right-turning traffic can lock the flow and bring the entire flow to a halt. One way of

dealing with heavy right-turning traffic is to incorporate a separate right-turning phase in the signal

scheme which result in a long signal cycle. Another solution is to ban the turning movement altogether.

Prohibition of right-turning movement can be established only if the existing street system is capable of

accommodating an alternative routing.

One-way Streets

As the name itself implies, one-way streets are those where traffic movement is permitted in only one

direction. As a traffic management measures intended to improve traffic flow, increase the capacity and

reduce the delays, one-way streets are known to yield beneficial results. They afford the most immediate

and the least expensive method of alleviating the traffic conditions in a busy area. In combination with

other methods such as banned turning movements, installation of signals and restrictions on loading and

waiting, the one-way street system is able to achieve great improvement in traffic conditions of congested

areas.

Whenever a system of one-way streets is introduced, it is imperative that proper signs should be put up to

foster safe and efficient traffic. 'No entry' signs are needed at all terminal points of the one-way streets. At

the entrances and exits of all intersections within the scheme, 'one-way' and/or 'two-way' traffic signs

should be displayed. It may be necessary to put up 'No left turn' and 'No right turn' signs at some

junctions.

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Advantages

Reduction in the points of

conflict

Increase capacity

Increase speed

Facilitating the operation

of a progressive signal

system

Improve in parking system

Elimination of dazzle and

head on collision

Tidal-flow operations

One of the familiar characteristics of traffic flow on any street leading to the city center is the imbalance

in directional distribution of traffic during the peak hours. For instance, the morning peak results in a

heavy preponderance of flow towards the city center, whereas the evening peak brings in heavier flow

away from the city center. In either case, the street space provided for the opposing traffic will be found to

be in excess. This phenomenon is commonly termed as "tidal flow". One method of dealing with this

problem is to allot more than half the lanes for one direction during the peak hours. This system is known

as "tidal flow operation", or "reverse flow operation".

Closing Side-streets

Figure 3Four legged intersection and conflict points

Figure 4 Tidal flow operation

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A main street may have a number of side-streets where the traffic may be very light. In such situations, it

may be possible to close some of these side-streets without affecting adversely the traffic, and yet reap a

number of benefits.

Exclusive Bus Lanes

A recent innovation in traffic management practice in some of the Cities is to reserve a lane of the

carriageway exclusively for but traffic. This is possibly only in situations where the carriageway is of

adequate width and a lane can be easily spared for the buses. This implies that there should be at least 3

lanes in each direction. For reasons of convenience of alighting and embarking passengers at the curb, the

exclusive lane has to be adjacent to the curb.

2.2 Calculation of traffic volume

Traffic volume is the number of the vehicles crossing a section of road per unit time at any selected

period. Traffic volume is used as a quantity measure of traffic flow. A complete traffic volume study

includes the classified volume study by recording the volume of various movements and the distribution

on different lanes per unit time. The volume of different type is usually converted into Passenger Car Unit

(PCU).

NRS 2070

Table 1PUC factors (Source: NRS 2070)

SN Vehicle type Equivalency factor

1 Bicycle, motorcycle 0.5

2 Car, auto rickshaw, SUV, light van and

pick up

1.0

3 Light (mini), truck, tractor, rickshaw 1.5

4 Truck, bus, minibus, tractor with trailer 3.0

5 Non-motorized carts 6.0

The objectives and uses of traffic volume studies are:

Traffic volume study is generally accepted as true measure of the relative importance of roads and

in deciding the priority for improvement and expansion.

Traffic volume study is used in planning, traffic operation and control of existing facilities and

also for planning and designing the new facilities.

Traffic volume study is used in the analysis of traffic patterns and trends

Classified traffic volume study is useful in structural design of pavements, in geometric design

and in computing roadway capacity. Volume distribution study is used in planning one way

streets and other regulatory measures.

Turning movement study is used in the design of intersections, in planning signal timings,

channelization and other control devices.

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Pedestrian traffic volume study is used for planning sidewalks, cross walks, subways and

pedestrian signals.

Types of Traffic Volume:

Average annual traffic flow: expressed in vehicle per year.

Annual Average Daily Traffic (AADT): expressed in vehicles per day. It is (1/365) th of the

total annual traffic flow. Total number of vehicles passing the site in a year is divided by 365

days. All vehicles are converted into passenger car unit.

Average Daily traffic (ADT): If the flow is not measured for all the 365 days, but only for few

days (less than one year) the average flow is known as Average Daily Traffic (ADT).

Average Annual Weekday Traffic (AAWT): is the average 24 hour traffic volume occurring on

weekdays over a full year.

Average weekday traffic: is an average 24 hour traffic volume occurring on weekdays for some

period less than one year, such as one month or one season.

Hourly flow: vehicle/hour, peak hour volume.

Variation in traffic flow and accuracy of counts

Traffic counts carried out over a very short time period can produce large errors because traffic flows

often have large hourly, daily, weekly, monthly and seasonal variations. These variations are described in

the following sections.

Hourly variations

An example of hourly traffic variation throughout one day is shown below. In this example major traffic

flow occurs between 05 and 21 hours. In practice traffic counts will usually be carried out for 12, 16 or 24

hour time periods. Typically, in tropical countries, a 12 hour traffic count (example from 6:00 to 18:00)

will measure approximately 80 % of the day’s traffic whereas a 16 hour count (example from 6: 00 to

22:00) will measure over 90 percent.

In order to obtain estimates of 24 hour flows from counts of less than 24 hours duration, it is necessary to

scale up the counts of shorter duration according to the ratio of flow obtained in 24 hours and the flows

measured in the shorter counting period.

Scale factor (converting a partial day’s count into a full day’s traffic count)

( ) ( )

( )

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Figure 5 Hourly Variation

For reasons of statistical analysis, HCM (1997) suggests using 15 minutes for most operational and design

analyses. The relationship between hourly volume and the maximum rate of flow within the hour is

defined as the peak hour factor.

For 15 minutes periods—the maximum value of the PHF 1.0 which occurs when the volume in each 15

minutes period is equal, the minimum value is 0.25 which occurs when the entirely hourly volume occurs

in one 15 minute interval.

Daily and weekly variation

The day to day traffic flows tend to vary more than the week to week flows over the year. Hence large

errors can be associated with estimating average daily traffic flows (and hence annual traffic flows) from

traffic counts of only a few days duration, or which exclude the weekends. Thus there is a rapid increase

in the accuracy of the survey as the duration of the counting period increases up to one week. For counts

longer than one week, the increase in accuracy is less pronounced.

Figure 6 Daily Variation

Monthly and seasonal variation

Traffic flows will rarely be the same throughout the year and will usually vary from month to month and

from season to season. The seasonal variation can be quite large and is caused by many factors. For

0

200

400

600

800

1000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hourly variation of traffic flow

0

2000

4000

6000

8000

SAT SUN MON TUES WED THURS FRI

Daily variation of traffic volume

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example, an increased traffic flow usually occurs at a harvest time, and a reduced traffic flow is likely to

occur in a wet season.

To reduce error in the estimated annual traffic data caused by seasonal traffic variations, it is desirable to

repeat the classified traffic count at different times of the year. A series of weekly traffic counts repeated

at intervals throughout the year will provide a much better estimate of the annual traffic volume than a

continuous traffic count of the same duration.

An example of seasonal variation is shown below. For one week traffic count carried out each month. A

seasonal factor (SF) of unity indicates average flow. A seasonal factor greater than unity, indicates a

higher proportion of traffic than the average. It can be seen that the traffic is lower than average in

December, January, February, July and August. The variation in flow for different classes of vehicle may

not be the same and this will be revealed in the classified traffic survey.

Figure 7Seasonal variation

Problem 2.2: The following counts were taken on an intersection

approach during the morning peak hour. Determine (a.) the hourly

volume, (b) the peak rate of flow within the hour and (c) the peak hour

factor.

Example 2.3: The following traffic count data were taken from a

permanent detector location.

Month

2 .No. of weekdays in

Month(days)

3.Total days in

Month(days)

4.Total Monthly

volume(vehs)

5.Total weekday

volume(vehs)

Jan 22 31 200000 170000

Feb 20 28 210000 171000

Mar 22 31 215000 185000

Apr 22 30 205000 180000

May 21 31 195000 172000

0

500

1000

1500

Jan Feb March Apr May Jun Jul Aug Sep Oct Nov Dec

Seasonal variation of traffic flow

Series1

1.50 1.31

0.92 0.80 0.83

1.07 1.13 1.25

0.96

0.71 0.84

1.31

0.00

0.50

1.00

1.50

2.00

Jan Feb March Apr May Jun Jul Aug Sep Oct Nov Dec

seasonal factors

Time Period Volume

8:00-8:15 AM 150

8:15-8:30 AM 155

8:30-8:45 AM 165

8:45-9:00 AM 160 snlkh

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14

Jun 22 30 193000 168000

Jul 23 31 180000 160000

Aug 21 31 175000 150000

Sep 22 30 189000 175000

Oct 22 31 198000 178000

Nov 21 30 205000 182000

Dec 22 31 200000 176000

From this data, determine (a) the AADT, (b) the ADT for each month, (c) the AAWT, and (d) the AWT

for each month, from this information, what can be discerned about the character of the facility and the

demand it serves?

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Traffic Engineering and Management /[email protected] 15

Example 2.1: Following table shows the classified manual vehicle count of a 14th

February 2015 for three hour at Pepsikola - Manohara - Thimi - Hanumante - Sallaghari

Road. Determine the AADT in term of PCU with given data: the three hour traffic figures out about 20% of the total traffic at that day, Lane factor for FR 0.6.

Result of Classified Manual Vehicle Count

Start Date:

Location: Pepsikola ,

Road Link: Pepsikola – Manohara

Note:Direction a: Pepsikola - Manohara

Station:

Name of Road: Pepsikola - Manohara - Thimi - Hanumante - Sallaghari Road Direction b:Manohara-Pepsikola

Station No.:

Seasonal Variation Factor for the Month of Feburary: 1.31

Surveyed By:

Date: 14th Feburary

2015

Date Start Time

(Hrs)

Volume of Vehicles

Truck Bus Car

Motor

Cycle

Utility

Vehicle Tractor

Three

Wheeler Rickshaw

Four Wheel

Drive/Jeep,Van

Power

Tiller Total

Heavy Light Mini Micro

A b a b a b a b a b a b a b a b a b a b a b a b a b

14th Feburary,

2015

12:30-1:30 2 9 13 13 12 17 23 194 203 8 7 1 6 7 247 268 515

1:30-2:30 6 2 14 6 10 12 1 32 30 183 188 9 9 1 1 1 1 9 10 1 1 266 261 527

2:30-3:30 4 5 37 52 41 57 98

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CHAPTER THREE: TRAFFIC CHARACTERISTICS

3.1 Basic traffic character istics - Speed, volume and concentration.

Traffic flow is very complex. It requires more than causal observation while driving on a freeway to discover

that as traffic flow increases, there is generally a corresponding decrease in speed. Speed also decreases, when

vehicles tend to bunch together for one reason or another. Such analysis includes transverse and longitudinal

distribution of vehicles, distribution over time. Thus the provision of the theoretical consistent quantitative

technique by which relevant dimensions of vehicular traffic can be modeled, forms the basis of traffic

analysis.

Traffic flow is a stochastic process, with random variations in vehicles and driver characteristics and their

interactions. The theory of traffic flow can be defined as a mathematical study of the movement of vehicles

over road network. The subject is a mathematical approach to define, characterize and describe different

aspects of vehicular traffic. The development of the topics has taken inspiration from the various field of

knowledge such as, statistics, applied mathematics, psychology and operation research etc.

Approaches to understanding traffic flow: Three main approaches to the understanding and quantification

of traffic flow. The first being the macroscopic based on the analogies as fluid flow. This approach is most

appropriate for studying steady state of flow and hence best describes efficiency of the system. The second is

microscopic approach that consider the response of each individual vehicle in a disaggregate manner. In this

case, individual driver-vehicle combination is examined, and therefore is extensively used in highway safety

work. The third is the human-factor approach, which basically tries to define the mechanism by which an

individual driver and the vehicle locate oneself with reference to another vehicle and the highway guidance

system.

Vehicle flow on transportation facilities may be classified into two categories:

Uninterrupted flow: it occurs on the facilities that have no fixed elements, such as traffic signals, external to

traffic stream , that cause interruption to traffic flow.

Interrupted flow: it occurs on transportation facilities that have fixed elements causing periodic interruptions

to traffic flow. Such elements are traffic signals, stop signs, and other types of controls. These devices cause

traffic to stop periodically.

It should be noted that uninterrupted and interrupted traffic flow are terms to describe the facility and not the

quality of flow.

Speed (v)

It is defined as the rate of motion, as distance per unit time, generally km/h. or m/sec. There is a wide

distribution of individual speed in a traffic stream, an average speed is considered. If travel time t1, t2, t3 .

…… tn, are observed from n vehicles traveling a segment of length L, the average travel speed is:

n

i

i

n

i

i

s

t

nL

n

t

Lv

11

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Example3.1: Three vehicles are traversing a 1.5 km segment of a highway and following observation is made:

What is the average travel speed of the vehicle?

Vehicle A: 1.2 min. (72 sec.)

Vehicle B: 1.5 min. (90 sec.)

Vehicle C: 1.7 min. (102 sec.)

The average travel speed calculated is referred as the space mean speed (vs). It is called ―space‖ mean speed

because the use of average travel time essentially weights the average according to length of time each vehicle

spends in space.

Another way of defining ―average speed‖ of traffic stream is by finding the time mean speed (vt). This is the

arithmetic mean of the measured speeds of all vehicles passing, say, a fixed roadside point during a given

interval of time, in which case, the individual speeds are known as ―spot” speeds.

n

v

v

n

i

i

t

1

Where, vi, is the spot speed, and n is the number of vehicles observed.

Volume (q)

Volume and rate of flow are two different measures. Volume is the actual number of vehicles observed or

predicted to be passing a point during a given time interval. The rate of flow represents the number of vehicles

passing a point during a time interval less than one hour, but expressed as an equivalent hourly rate.

Density or concentration

It is defined as the number of vehicles occupying a given length of lane or roadway, averaged over time

usually expressed as vehicles per km. direct measurement of density can be obtained through aerial

photography, but more commonly it is calculated from the equation if speed and rate of flow are known,:

kvq *

Where, q = rate of flow veh. /hr)

v = average travel speed, m/sec

k = average density (veh/km)

3.2Relationship between Flow, Speed and Concentration

q = rate of flow veh. /hr

v = average travel speed, m/sec

k = average density (veh/km)

Then,

=

Now Density, k =

Hence q=k*v

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Analysis of Speed, Flow, and Density Relationship

It has been assumed that a linear relationship exists between the speed of traffic on an uninterrupted traffic

lane and the traffic density, as shown in figure below. Mathematically it is represented by:

BkAv

Where, v is the mean speed of the vehicle.

k is the average density of vehicles veh/km.

A and B are empirically determined parameters

We know,

B

vv

B

A

B

vAvkvq

BkAkkvq2

2

)(

At almost zero density, the free mean speed equals to A, and at almost zero speed, the jam density equals A/B.

The maximum flow occurs at about half the mean free speed and is equal to A2/4B .

The theoretical relationship between flow and density on a highway lane, represented by a parabola. The flow

increases from zero to its maximum value, the corresponding density of this flow is optimum density (ko).

From this point onward to the right, the flow decreases as the density increases. At the jam density (kj), the

flow is almost zero.

Density, veh/km

Me

an

sp

ee

d, km

/h

Density, veh/km

Flo

w, ve

h/h

Flow, veh/h

Me

an

sp

ee

d, km

/h

A

A/B

A2 /4B

V=A-Bk

A/2B

A/B

A

A/2

a) b) c)

Speed -Flow-Density curves

A2/4B

Figure 8Speed-Flow-Density curves

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Greenshields’ Model: in 1935, Greenshields developed a model based on empirical studies.

.....(ii).................... }{ or,

)( or, )( Then,

)........(i.......... )(Q Then,

or, *Q that know We

ionconcentrat

ionconcentrat jamming

conditions flow freefor speedmean space

speedmean space

)(

2

2

s

sf

j

js

sjssfsf

sj

sf

sfs

j

sf

sf

s

s

j

sf

s

j

sf

sfs

vv

KKvQ

vKvvQvv

Q

K

vvv

KK

vKv

v

QKKv

K

K

v

v

KK

vvv

Differentiating the equation (i) with respect to concentration, we can get the value of concentration

corresponding to the maximum flow.

i).......(ii.................... 2

Then,

02

jKK

jK

ksfvsfv

dK

dQ

To obtain the speed corresponding to the maximum flow, the equation (ii) is differentiated with respect to vs.

......(iv).................... 2

or,

02

sfvsv

sfv

svjKjK

sdv

dQ

(v).................... 42

*2

maxK * Q maximumfor )(max :Therefore

jKsfvjKsfv

imumQforsvimumQ

Relation between time mean speed and space mean speed:

Considering a stream of traffic with a total flow of Q, consisting of subsidiary stream with flows q1, q2, q3

….qc and speeds v1, v2, v3, ……vc.

c

i

iqqqqQ1

c321 q..........................

For the subsidiary stream with flow q1 and speed v1:

Average time headway = 1/q1

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Distance traveled in that time = v1/q1

The density of the stream in space (the number of vehicles per unit length at any instant is given by:

ctv

qk

i

ii ......3 ,2 ,1 ,

The total concentration:

c

iikK

1

The time mean speed s defined as:

c

i

iit

Q

vqv

1

The space mean speed is defined as:

c

i

iis

K

vkv

1

Relationship between space mean speed and time mean speed

s

sst

vvv

2

Example 3.2: Assuming a linear speed-density relationship, the mean free speed is observed to be 85 km/h

near zero density, and at the corresponding jam density is 140veh/km. Assume that, the average length of

vehicles is 6m.

Write down the speed-density and flow-density equations.

Draw the v-k, v-q and q-k diagrams indicating critical values.

Compute speed and density corresponding to flow of 1000 veh/h.

Example 3.3: Speed observations from a radar speed meter have been taken, giving the speeds of the

subsidiary streams composing the flow along with the volume of traffic of each subsidiary stream. The

readings are as under.

Speed range 2-5 6-9 10-13 14-17 18-21 22-25 26-29 30-33 34-37 38-41 42-45 46-49 50-53 54-57 58-61

Volume (qi) 1 4 0 7 20 44 80 82 79 49 36 26 9 10 3

Calculate: a) time mean speed b) space mean speed c) variance about space mean speed

Example 3.4: The speed density relationship of traffic on a section of a freeway lane was estimated to be

Vs = 18.2 ln(220/k)

a) What is the maximum flow, speed, and density at this flow?

b) What is the jam density?

Example 3.5: Determine the maximum flow for the free flow speed of 80 kmph. The aerial photograph shows

that average center to center spacing of two vehicle during jam (i.e. velocity is zero) is found to be 6.5 m.

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CHAPTER FOUR: TRAFFIC MEASUREMENT AND ANALYSIS

4.1 Volume Studies

Definition

Traffic volume is the number of the vehicles crossing a section of road per unit time at any selected period.

Traffic volume is used as a quantity measure of traffic flow. Unit used for this is vehicle/day; vehicle/hour etc.

A complete traffic volume may include the classified volume study by recording the volume of various types

of vehicle, distribution by direction and lane and turning movements.

The volume of different type is usually converted into Passenger Car Unit (PCU).

NRS 2070

SN Vehicle type Equivalency factor

1 Bicycle, motorcycle 0.5

2 Car, auto rickshaw, SUV, light van and

pick up

1.0

3 Light (mini), truck, tractor, rickshaw 1.5

4 Truck, bus, minibus, tractor with trailer 3.0

5 Non-motorized carts 6.0

Objectives:

It is the true measure of relative importance of roads, which is important for improvement and expansion.

Traffic volume is used in planning, traffic operation/control of existing facilities and for planning new facilities.

Classified volume is used for structural design of pavements.

It is used to analyze traffic pattern and trends.

It is used for design intersections, signal timings, canalizations, and other control devices. For the determination of one-way street or other regulatory measures.

Pedestrian traffic volume is uses for planning and design of sidewalks, cross walks, subways, and pedestrian signals.

Hourly traffic volume varies considerably during a day. Peak hour is much higher than average hourly

volume. Daily traffic varies in a week and also with season.

Types of traffic counts:

Short term counts:

For determining traffic flow in peak hours.

To measure saturation flow at signalized intersection

Count for full day

To determine hourly fluctuation of flow

Used intersection counts

Count for full week:

To determine hourly and daily fluctuation of flow

For traffic survey in urban highways.

Continuous count:

To determine fluctuation daily, weekly, seasonal and yearly flow.

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To determine annual traffic growth rate

Very commonly used in developed countries at selected sections.

Methods of traffic counts:

Manual count

Combined Manual and mechanical counter

Automatic devices

Photographic Method.

Moving observer method

Manual Count:

The prescribed record sheet is provided for manual count. Vehicles are counted by the method of five-dash

system.

Date: Road classification: Klometrage /mileage

Direction:

Vehicle type

Hour

Car, Jeep,

Van

Bus Micro

bus

Truck Three-

wheeler

Motor

bike

Cycle

8-9

9-11

11-12

It is more desirable to record traffic in each direction of travel separately. The data can be summarized for

each hour

of day.

Advantages of manual method:

Vehicle classification, type and occupants

Record of turning and straight going vehicle

Directional breakdown

Check of automatic count Data easy to analyze Suitable for short and non-continuous count.

Pedestrian count can also be done.

Enable to record unusual conditions

Figure 9Manual Traffic count snlkh

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Disadvantages:

Costly

Continuous counting is not feasible.

Number of team member depends on the number of lane, total volume, complexity of area .

Equipment and tools:

Watch, pencils, erasers, blank field data sheets and a clipboard

Mechanical tally counters and electronic count boards can also be used which can be directly

downloaded to computers.

Manual count at intersections:

Field data sheet can be modified to suit the particular requirements of any intersection. Traffic enumerators

needed to be posted on each arm of the intersection. The count of traffic on each arm should be broken down

into three categories- left turning, right turning and straight ahead traffic.

Combined Manual and Mechanical Method

An example of a combination of manual and mechanical method is the multiple pen recorders. The chart

moves continuously at the speed of a clock. Different pens record the occurrence of different events on the

chart. Particular pen may record specific type of vehicle. Advantage of this type is:

Classification and count is done simultaneously

Time headway can be determined

Automatic device

The automatic devices consist of equipment for detecting the passage or presence of and another for recording

the count. The sensor transmits some form of electric impulse which activates the accumulating register or

record chart.

Sensors (detectors):

1) Pneumatic tube (road tube): flexible tube with one end sealed is clamped to the road surface at right angles

to the pavement. Other end of the tube is connected to a diaphragm actuated switch. When an axle of the

vehicle crosses the tube a volume of air gets displaced thus creating a pressure which instantaneously closes

the electric contact through the switch. Two such contacts result in one count for the two axle vehicle. They

are cheap but it is difficult to fix on gravel surface and they are likely to be damaged by tractors and are easily

pilfered by vandals. They cannot detect vehicles by lane.

Figure 10 Combined Mechanical and Manual Method

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2) Electric contact: A pair of steel strip is contained in a rubber pad which is buried beneath the road surface.

By the load of the vehicle, steel strips come into contact with each other and cause the electric current to flow.

3) Co-axial cable: A co-axial cable is clamped across the road surface, with the capability of generating

signals with the passage of axles. These signals actuate a transistorized counter. They are more reliable then

above-mentioned devices.

4) Photo-electric: A source of light is installed on the one side of the road, which emits a beam of light across

the road. At the other end a photo-cell which can distinguish the light beam and its absence, is fixed. By the

passage of vehicle, photo-cell records the obstruction to the light beam. There may be error due to passage of

vehicle at the same time in different lane.

5) Radar: A Radar (Doppler Effect) may detect the vehicle moving at a speed. When a moving object

approaches or recedes from the sources of signals, the frequency of the signal received back from the moving

object will be different from the frequency of the signal emitted by the source. This difference in two

frequencies causes the detection of the moving object. The initial cost is high but it is reliable and accurate.

6) Infra-red: Infrared sensors can detect the heat radiated from a vehicle or can react to the reflection from the

vehicle of infra-red radiation emitted by the sensors.

7) Magnetic: the disturbance caused by the passage of vehicle to the magnetic field, is taken as the basis of

sensing. Magnetic field is provided by a wire coil, which is buried beneath the road surface.

Recording Mechanism:

The signals generated by the automatic sensors can be recorded by the various methods:

1) Counting register: it is simply an accumulating counter, which indicates the number of the vehicle.

Readings must be taken before and after the counting period.

2) Printed output: this device prints the accumulated totals at regular interval of time on a roll of paper.

3) Electronic system: they are modern system, which can record data directly on floppies or other magnetic

disk.

Video Photographic method:

It gives the permanent record of volume counts. Its analysis can be done at office by replaying the cassette.

Presentation and analysis of traffic volume data

Data collected during the traffic volume study are sorted out and are presented in any of the following forms

depending upon requirements:

Average Annual Daily Traffic (AADT): it is 1/365th

of the total annual traffic flow. It is expressed in terms

of PCU and used for determining importance and future development of the road.

Trend Chart: it shows the volume trends over the period of years. By extrapolating the trend we can estimate

the future volume prediction.

Variation Chart: for the presentation of hourly, daily, weekly, seasonal variations such charts are prepared.

They are useful to determine facilities and regulations for the peak hour requirements.

Traffic flow at intersection shown by thick lines: for the intersection design and control measures.

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Traffic Flow Maps: along the routes of the road

network, are useful for the graphical presentation

for the volume study. Traffic volume distribution on

the road network can easily be noticed.

Ho

urly T

raffic

Vo

lum

e, %

of A

DT

0 20 40 60 80 100

Numbers of hours in one year

with traffic volume

exceeding that shown

30

th h

igh

est h

ou

r

30th

Highest Hourly Volume (design hourly

volume): It is the hourly volume that will be

exceeded only 29 times in a year and all other

hourly volumes of the year will be less than this

value. For the economic point of view, the highest

hourly volume is not accepted for designing of the

facilities. And the annual average hourly volume

will not be sufficient during considerable period of

year. So for designing traffic facilities, the

congestion only 29 hours in the year is permissible.

Thus the 30th

highest hourly volume is generally

taken as the hourly design volume.

1050

450

600

100

700

300

Traffic flow at intersection

0

500

1000

1500

2000

2500

3000

3500

4000

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

Tota

l num

ber

of

tra

ffic

flo

w

Number of 15 min duration

15 minutes total traffic flow

Traffic flow map

Figure 11Graphical Representation of Traffic data

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4.2 Speed studies

Definition of terms

Speed is a factor influencing traffic flow on existing roads. Speed studies are essential for:

Traffic operation like sign location and timings, establishing speed zones etc.

Geometric design of elements like curvatures, super elevation, stopping sight distance etc.

Spot speed: it is an instantaneous speed of a vehicle at a specific location.

Running speed: It is the average speed of vehicles along a given section of road excluding delays at

controlled intersection.

Running speed = length of course/running time = length / (journey time- delay time)

It is useful for assessing traffic capacity of roads.

Journey speed: it is average speed of vehicles along a route including all delays, but excluding all voluntary

stoppages. It is useful in urban areas for measuring time adequacy an existing road network, for assessing the

efficiency of the improvement measures.

Journey speed = length of route / total journey time including delays

Average speed: average spot speed of several vehicles passing a specific section is termed as average speed.

Application:

For the traffic control and regulation, in geometric design, accident studies, studying traffic capacity etc.

Effect of traffic flow constraints like bridge and intersection

Spot speed is affected by physical of road like pavement width, curve, sight distance and grade.

There are two types of average speed: Space mean speed and time mean speed.

Space mean speed: Average speed of vehicles over a certain length of road at a given time. This is obtained

from the observed time of the vehicles over a relatively long stretch of the road.

Space mean speed (kmph),

n

ii

s

t

dnV

1

6.3

n=Number of individual vehicle observation; d - Length of the road section. ti - observed travel time in sec

for the i th vehicle to travel d m.

Time mean speed: it represents speed distribution of vehicles at a point on the roadway and it is the average

of instantaneous speed of observed vehicles at the spot.

n

V

V

n

ii

t

1

Vt is time mean speed (km/h); Vi observed instantaneous speed

of the i th vehicle kmph; n no of vehicles observed.

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Spot Speed study:

Uses:

Geometric design of roads;

Regulation and control of traffic operation; Anglicizing the causes of accidents;

Before and after study of improvement projects;

Determining the problems of congestion in the road section;

Capacity study.


Methods of Spot speed measurement:

Methods available for the measuring spot speed can be grouped as follows:

Those who require observation of time taken be the vehicle to cover a known distance;

Direct timing procedure;

Enoscope

Pressure contact tube

Radar speed-meter which automatically records the instantaneous speed;

Photographic method

General consideration for the site selection foe spot speed measurement:

Location selection should be according to the specific purpose;

Minimum influence to the traffic flow and their speed by the survey team and equipment;

Generally straight, level and open section should be selected.

Recommended base length:

Average speed of traffic stream, km/h Base length

Less than 40 27

40-65 54

Greater than 65 81

a) Direct timing procedure for the spot speed determination:

Simple method

Two reference points are marked on the pavement at a suitable distance apart and an observer starts and stops an accurate stopwatch as a vehicle crosses these two marks.

From the known distance and measured time intervals spot speed is calculated;

Large effects may occur due to the parallax effect;

Reaction of individual observer may affect the result.

One observer stands at the first reference point and gives signal to the observer standing at last reference point (with stopwatch).

Figure 12 Stopwatch spot speed study layout

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b) Enoscope

It is a simple device consisting of L-shaped mirror box, open at both ends. It has a mirror set fixed at 45

degree to the arms of the instrument as in figure.

50 m

Light for night

Figure 13: Enoscope method

c) Pressure contact tubes

In this method detectors are used to indicate the time of entering and leaving the base length by the vehicle.

d) Inductive loop detector

Two wire loops are inserted in the pavement at known distance apart and radio frequency at 85-115 kHz is fed

to the circuit tuned to avoid electric interferences. When the vehicle passes over the loop it causes shift of

phase in frequency thus recording the vehicle presence.

e) Radar speed meter: This automatic device works on the Doppler principle that the speed of a moving

body is proportional to the change in frequency between the radio wave transmitted to the moving body and

the radio wave received back. It directly measures speed.

Figure 14Pressure contact tubes

Figure 15Traffic Police using Radar Gun Meter

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f) Photographic and video camera method

Time-lapse camera photography has been used to determine the speed of the vehicles. In this method,

photographs are taken at fixed intervals of time on a special camera. By projecting the film on the screen, the

passage of any vehicle can be traced with reference to time.

Video camera also can be used to measure the speed of the vehicle.

Presentation and analysis of spot speed data

Spot speed depends on factors like volume and composition of traffic, geometric layout, and condition of the

road, environmental conditions, human and vehicle characteristics etc. Careful consideration is necessary

while presenting the data.

Tabular presentation: grouping of spot speeds into speed cases to facilitate esay computation.

Graphical presentation: (Histogram and cumulative frequency curves)

Modal speed: peak of the frequency curve. (mode of the distribution)

Median Speed: 50th

percentile speed

98th

percentile speed: below this speed 98% of vehicles move, and it is taken as design speed for the

geometric design.

85th

percentile speed: 85% of the vehicles move below this speed. It is used to establish upper speed limit

for traffic management. It is taken as limit of safe speed in the road.

15th

percentile speed: 15% of vehicles move below this speed. It is used for determining minimum speed

limit for major highways.

Arithmetic mean or average spot speed: Summation of all variable speed divided by the number of

observations.

Spot speed observation table (say the stretch of the road section L=50 m):

Observation

number 1 2 3 4 5 … …..

Observed time 3.52 3.45 2.85 3.25 2.65 ….. ….

Speed (km/h) 51.14 52.2 63.2 55.4 67.9 …. …

Grouping of data and presentation:

Large amount of data could be presented by arranging them in a frequency table . First data should be

grouped into suitable class interval. Size of class interval:

N

Rangei

10log22.31 Where, i is the class interval, N is the number of observations.

Parameters of Distribution

The frequency table, histogram and the cumulative frequency curve give only the rough idea of the

distribution and their inherent characteristics. An accurate idea about distribution can be expressed from the

parameters of distributions .

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Histogram: vertical axis: percentage

frequency; horizontal axis: class-limits.

The cumulative percentage frequency

diagram:

The Pace: Another Measure of Central

Tendency The pace is a traffic engineering

measure not commonly used for other

statistical analyses. It is defined as the 10

kmph increment in speed in which the

highest percentage of drivers is observed. It

is also found graphically using the

frequency distribution curve. The solution

recognizes that the area under the

frequency distribution curve between any

two speeds approximates the percentage of

vehicles traveling between those two

speeds, where the total area under the curve

is 100%.

The pace is found as follows: A 10 kmph

template is scaled from the axis. Keeping

this template horizontal, place an end on

the side of the curve a move slowly along

the curve. When the right side of the

template intersects the right side of the curve, the pace located. This procedure identifies the 10 kmph

increment that the peak of the curve; this contains the most area and, the highest percentage of vehicles.

Percent Vehicles within the Pace. The pace itself is a measure of the center of the distribution. The

percentage of vehicles traveling within the pace speeds is a measure of both central tendency and dispersion.

The smaller the percentage of vehicles traveling within the pace, the greater the degree of dispersion in the

distribution. The percent of vehicles within the pace is found graphically using both the frequency distribution

and cumulative frequency distribution curves. The pace speeds were determined previously from the

frequency distribution curves. Lines from these speeds are dropped vertically to the cumulative frequency

distribution curve. The percentage of vehicles traveling at or below each of these speeds can then be

determined from the vertical axis of the cumulative frequency distribution curve

Example 4.1: Three cars with speed 20kmph, 40kmph and 60kmph travelling length D. Determine the space

mean speed and time mean speed.

Example 4.2: Twenty five spot speed observations were taken and were as under (km/h):

50, 40, 60, 54, 45, 31, 72, 58, 43, 52, 46, 56, 43, 65, 33, 69, 34, 51, 47, 41, 62, 43, 55, 40, 49

Calculate: a) time mean speed, b) space mean speed, and c) verify the relation between them.

30.00 40.00 50.00 60.00

speed, km/h

0.00

5.00

10.00

15.00

20.00

% F

requ

ency

LLR Smoother

Figure 16Presentation of Speed data

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Example 4.3: Consider the following spot speed data, collected from a freeway site operating under free-flow

conditions:

a) Plot the frequency and cumulative frequency curves for these data.

b) Find and identify on the curves: medium speed, modal speed, pace, percent vehicles in pace.

c) Compute the mean and standard deviation of the speed distribution.

Speed Group (kmph) Number of vehicles observed (N)

15-20 0

20-25 3

25-30 6

30-35 18

35-40 45

40-45 48

45-50 18

50-55 12

55-60 4

60-65 3

65-70 0

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CHAPTER FIVE: SPEED STUDIES

5.1Head ways and Gaps

Spacing and headway are two additional characteristics of traffic streams. Spacing (s) is defined as the

distance between successive vehicles in a traffic stream as measured from front bumper to front bumper.

Headway (h) is the corresponding time between successive vehicles as they pass a point on a roadway. Both

spacing and headway are related to speed, flow rate and density.

hourvehh

q

v

msh

kmvehk

/;sec, )(headway average

3600

sec,m/sec ),( speed average

),( spacing average

/;m (s), spacing average

1000

Spacing of vehicles in a traffic lane can generally be observed from aerial photographs. Headway of the

vehicles can be measured using stopwatch observations as vehicles pass a point on a lane.

20 40 60 80

Speed, km/h

Min

imum

spa

cing

, m

10

20

30

40

60

80

Figure: Variation of Min spacing and headway with speed

0

Min

hea

dway

, sec

100

0

1

2

3

Headway, Sec

Spacing, m

Spacing (m) or

headway (sec)

L, m

Clearance (m)

gap (sec)

Figure: Clearance-gap and Spacing-headway

Figure 17 Clearance-gap and Spacing- Headway

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Lane occupancy (R): is a measure used in freeway surveillance. If one could measure the lengths of vehicles

on a given roadway section and compute the ratio.

D

LR

i

section way road oflength

vehiclesof lengths of sum

R could be divided by the average length of a vehicle to give an estimate of density (k).

Lane occupancy (LO) can also be described as the ratio of the time that vehicles are present at a detection

station in a traffic lane compared to the time of sampling.

L

C

L= length of the vehicle

C= distance between the loops of detector

Loop detector

sv

CLt

T

tLO

0

0

n timeobservatio total

occupied isdetector vehicle timetotal

Density can be calculated by the formula:

CL

LOk

1000*

Lane clearance (c) and Gap (g) are related to the spacing parameter and headway. These four measurements

are shown in figure below. The difference between spacing and clearance is obviously the average length of a

vehicle in m. Similarly the difference between headway and gap is the time equivalence of average length of a

vehicle (L/v):

5.2 Uncontrol led Intersection

An intersection is a road junction where two or more roads either meet or cross at grade. This intersection

includes the areas needed for all modes of travel: pedestrian, bicycle, motor vehicle, and transit. Thus, the

intersection includes not only the pavement area, but typically the adjacent sidewalks and pedestrian curb cut

Where, g is the gap, m;

L is the mean length of vehicle, m;

c is the mean clearance, m;

h is the mean headway, sec;

v is the mean speed, m/sec

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ramps. All the road junctions designated for the vehicles to turn to different directions to reach their desired

destinations. Traffic intersections are complex locations on any highway. This is because vehicles moving in

different direction want to occupy same space at the same time. In addition, the pedestrians also seek same

space for crossing. Drivers have to make split second decision at an intersection by considering his route,

intersection geometry, speed and direction of other vehicles etc. A small error in judgment can cause severe

accidents. It causes delay and it depends on type, geometry, and type of control. Overall traffic flow depends

on the performance of the intersections. It also affects the capacity of the road. Therefore, both from the

accident perspective and the capacity perspective, the study of intersections are very important by the traffic

engineers.

Categories of Intersection

Intersection design can vary widely in terms of size, shape, number of travel lanes, and number of turn lanes.

Basically, there are four types of intersections, determined by the number of road segments and priority usage.

Priority Intersection: Occur where one of the intersecting roads is given definite priority over the other. The

minor road will usually be controlled by some form of sign marking, such as stop or yield sign; thus ensuring

that priority vehicles travailing on the main street will incur virtually no delay.

Space sharing intersection: Are intended to permit fully equally priority and to permit continuous movement

for all intersecting vehicle flows; example would be rotaries and other weaving areas.

Time Sharing Intersection: Are those at which alternative flows are given the right of way at different point

in time. This type of intersection is controlled by traffic signal or by police officer.

Uncontrolled intersection: are the most common type of intersection usually occurs where the intersecting

roads are relatively equal importance and found in areas where there is not much traffic shown in figure. At

uncontrolled intersection the arrival rate and individuals drivers generally determine the manner of operation,

while the resulting performance characteristics are derived from joint consideration of flow conditions and

driver judgment and behavior patterns. In simplest terms, an intersection, one flow of traffic ―seeks gaps‖ in

the opposing flow of traffic.

At priority intersections, since one flow is given priority over the right of way it is clear that the secondary or

minor flow is usually the one ―seeking gaps‖. By contrast at uncontrolled intersection, each flow must seek

gaps in the other opposing flow. When flows are very light, which is the case on most urban and rural roads

large gaps exist in the flows and thus few situation arise when vehicles arrive at uncontrolled intersection less

than 10 second apart or at interval close enough to cause conflicts. However when vehicles arrive at

uncontrolled intersection only a few second apart potential conflicts exist and driver must judge their relative

time relationships and adjusts accordingly.

Generally one or both vehicles most adjust their speeds i.e. delayed somewhat with the closer vehicle most

often taking the right of way; in a sense, of course, the earlier arriving vehicle has ―priority‖ and in this

instance when two vehicles arrive simultaneous, the rule of the road usually indicate ―priority‖ for the driver

on the right. The possibility of judgmental in these, informal priority situation for uncontrolled intersection is

obvious. At an uncontrolled intersection, service discipline is typically controlled by signs (stop or yield signs)

using two rules two way stop controlled intersection (TWSC) and all way stop controlled intersection

(AWSC).

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Two-Way Stop-Controlled Intersection

Researchers rely on many specific definitions to describe

the performance of traffic operation systems. The clear understanding of such terminology is an important

element is studying two-way stop-controlled (TWSC) traffic operation system characteristics; defined as: One

of the uncontrolled intersections with stop control on the minor street shown in Fig. 4.

Characteristics of TWSC Intersections

At TWSC intersections, the stop-controlled approaches are referred to as the minor street approaches; the

intersection approaches that are not controlled by stop signs are referred to as the major street approaches. A

three-leg intersection is considered to be a standard type of TWSC intersection if the single minor street

approach is controlled by a stop sign. Three-leg intersections where two of the three approaches are controlled

by stop signs are a special form of uncontrolled intersection control.

Figure 20Traffic flow stream in two way stop controlled intersection source

Figure 19Two way stop controlled intersection

Figure 18uncontrolled intersection

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Flow at TWSC Intersections

TWSC intersections assign the right-of-way among conflicting traffic streams according to the following

hierarchy:

Rank 1 - The major street through and right-turning movements are the highest-priority movements at a

TWSC intersection. This movements shown Fig. 4 are 2, 3, 5, 6, 15 and 16.

Rank 2 - Vehicles turning left from the major street onto the minor street yield only to conflicting Major

Street through and right-turning vehicles. All other conflicting movements yield to these major street left-

turning movements. The movements on this rank are 1, 4, 13, 14, 9 and 12.

Rank 3 - Minor Street through vehicles yield to all conflicting major street through, right-turning, and left-

turning movements. The movements on this rank are 8 and 11.

Rank 4 - Minor Street left-turning vehicles yield to all conflicting major street through, right-turning, and left-

turning vehicles and to all conflicting Minor Street through and right-turning vehicles. The movements on this

rank are 7 and 10.

All-Way-Stop-Controlled Intersection (AWSC)

AWSC Intersection are mostly used approaching from all directions and is required to stop before proceeding

through the intersection as shown in Fig. 5. An all-way stop may have multiple approaches and may be

marked with a supplemental plate stating the number of approaches.

Figure 21All way stop controlled intersection

The analysis of AWSC intersection is easier because all users must stop. In this type of intersection the critical

entity of the capacity is the average intersection departure head way.

Secondary parameters are the number of cross lanes, turning percentages, and the distribution volume on each

approach. The first step for the analysis of capacity is select approach called subject approach the approach

opposite to subject approach is opposing approach, and the approach on the side of the subject approach is are

called conflicting approach.

Characteristics of AWSC Intersections

AWSC intersections require every vehicle to stop at the intersection before proceeding. Since each driver

must stop, the judgment as to whether to proceed into the intersection is a function of traffic conditions on the

other approaches. If no traffic is present on the other approaches, a driver can proceed immediately after the

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stop is made. If there is traffic on one or more of the other approaches, a driver proceeds only after

determining that there are no vehicles currently in the intersection and that it is the drivers turn to proceed.

5.3 Gap acceptance studies

A gap is defined as the gap in between the lead and lag vehicles in the target lane. For merging into an

adjacent lane, a gap is acceptable only when both lead and lag gap are acceptable. Drivers are assumed to have

minimum acceptable lead and lag gap lengths which are termed as the lead and lag critical gaps respectively.

These critical gaps vary not only among different individuals, but also for a given individual under different

traffic conditions. Most models also make a distinction between the lead gap and the lag gap and require that

both are acceptable. The lead gap is the gap between the subject vehicle and the vehicle ahead of it in the lane

it is changing to. The lag gap is defined in the same way relative to the vehicle behind in that lane.

Gap acceptance is an important element in most lane-changing models. In order to execute a lane-change, the

driver assesses the positions and speeds of the lead and following vehicles in the target lane and decides

whether the gap between them is sufficient. Gap acceptance models are formulated as binary choice problems,

in which drivers decide whether to accept or reject the available gap by comparing it to the critical gap

(minimum acceptable gap).

Basic Terminologies

“Gap” means the time and space that a subject vehicle needs to merge adequately safely between two

vehicles. Gap acceptance is the minimum gap required to finish lane changing safely. Therefore, a gap

acceptance model can help describe how a driver judges whether to accept or not.

Gap acceptance: The process by which a minor stream vehicle accepts an available gap to maneuver.

Critical gap: The minimum major-stream headway during which a minor-street vehicle can make a maneuver.

Lag: Time interval between the arrival of a yielding vehicle and the passage of the next priority stream

vehicle (Forward waiting time).

Headway: The time interval between the arrivals of two successive vehicles. Headway differs from gap

because it is measured from the front bumper of the front vehicle to the front bumper of the next vehicle.

Minimum Headway: The minimum gap maintained by a vehicle in the major traffic stream.

Follow-up time: Time between the departure of one vehicle from the minor street and the departure of the

next vehicle using the same gap under a condition of continuous queuing.

Delay: The additional travel time experienced by a driver, passenger or pedestrian.

Conflicting movements: The traffic streams in conflict at an intersection.

Capacity: The maximum hourly rate at which persons or vehicles can reasonably be expected to traverse a

point or uniform section of a lane or a roadway during a given time period under prevailing roadway, traffic,

and control conditions.

Gap Acceptance Model

Gap acceptance is one of the most important components in microscopic traffic characteristic. The gap

acceptance theory commonly used in the analysis of uncontrolled intersections based on the concept of

defining the extent drivers will be able to utilize a gap of particular size or duration. A driver entering into or

going across a traffic stream must evaluate the space between a potentially conflicting vehicle and decide

whether to cross or enter or not. One of the most important aspects of traffic operation is the interaction of

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vehicles with in a single stream of traffic or the interaction of two separate traffic streams. This interaction

takes place when a driver changes lanes merging in to a traffic stream or crosses a traffic stream. Inherent in

the traffic interaction associated with these basic maneuvers is concept of gap acceptance.

Figure 22Gap Acceptance Model

The subject vehicle tends to move from its current lane to target Lane, into the gap between 2 vehicles

travelling in the target lane. When a driver wants to do lane changing, the critical lead gap and the lag gap are

required to be acceptable for the driver. Otherwise, it is not safe for the driver to do the lane changing.

Critical Gap

The critical gap tcx for movement ―x‖ is defined as the minimum average acceptable gap that allows

intersection entry for one Minor Street or Major Street. The term average acceptable means that the average

driver would accept or choose to utilize a gap of this size. The gap is measured as the clear time in the traffic

stream defined by all conflicting movements. Thus, the model assumes that all gaps shorter than tcx are

rejected or unused, while all gaps equal to or larger than tcx would be accepted or used. The adjusted critical

gap tcx computed as follows.

cLTtcTtGcGtHV

PcHVtcbtcxt

Where, tcx = critical gap for movement ―x‖,

tcb = base critical gap from Table.

tcHV = adjustment factor for heavy vehicles

PHV = proportion of heavy vehicles

tcG = adjustment factor for grade

G = percent grade divided by 100,

tcT = adjustment factor for each part of a two-stage gap acceptance process

tcLT =critical gap adjustment factor for intersection geometry

Follow up Time

The follow up time tfx for movement ―x‖ is the minimum average acceptable time for a second queued minor

street vehicle to use a gap large enough admit two or more vehicles. Follow-up times were measured directly

by observing traffic flow. Resulting follow-up times were analyzed to determine their dependence on different

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arameters such as intersection layout. This measurement is similar to the saturation flow rate at signalized

intersection. Following table shows base or unadjusted values of the critical gap and follow up time for

various movements. Base critical gaps and follow up times can be adjusted to account for a number of

conditions, including heavy - vehicle presence grade, and the existence of two stage gap acceptance. Adjusted

Follow up Time computed as:

HVPfHVtfbtfxt

Where,

tfx = Follow-up time for minor movement x

tfb = Base follow-up time from table 1

tfHV = Adjustment factor for heavy vehicles

PHV = Proportion of heavy vehicles for minor movement

Table 3Adjustments to Base critical gap and follow up times

source

Conflicts

The traffic flow process at un-controlled intersection is

complicated since there are many distinct vehicular

movements to be accounted for. Most of this

movements conflict with opposing vehicular volumes.

These conflicts result in decreasing capacity, increasing

delay, and increasing potentials for traffic accidents.

Consider a typical four-legged intersection as shown in

Fig. The numbers of conflicts for competing through

movements are 4, while competing right turn and

through movements are 8. The conflicts between right turn traffics are 4, and between left turn and merging

traffic are 4. The conflicts created by pedestrians will be 8 taking into account all the four approaches.

Adjustment Values

tcHV 1.0, Two-lane major street

2.0, Four-lane major street

tcG 0.1, Movements 9 and 12

0.2, Movements 7, 8, 10 and 11

1.0, Otherwise

tcT 1.0, With two stage process

0.0, With single stage process

TcLT 0.7, Minor-street LT at T-intersection

0.0, Otherwise

tfHV 0.9, Two-lane major street

1.0, Four-lane major street

Movement

tcb, sec tfb, sec

Two-lane

Major Street

Four-lane

Major Street

Major LT

Minor RT

Minor TH

Minor LT

4.1

6.2

6.5

7.1

4.1

6.9

6.5

7.5

2.2

3.3

4.0

3.5

Table 2Base critical gap and follow up times source

Figure 23Conflicts at four legged intersection

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Diverging traffic also produces about 4 conflicts. Therefore, a typical four legged intersection has about 32

different types of conflicts.

Determine Conflicting Volume

Conflicts at an intersection are different for different types of intersection. The essence of the intersection

control is to resolve these conflicts at the intersection for the safe and efficient movement of both vehicular

traffic and pedestrians. The movements for determining conflict in four legged intersection are:

Major Street left turns seek gaps through the opposing through movement, the opposing right turn movement

and pedestrians crossing the far side of the minor street.

Minor street right turns seek to merge in to the right most lane of the major street, which contains through

and right turning vehicles. Each right turn from the minor street must also cross the two pedestrian’s path

shown.

Through movements from the minor street must cross all Major Street vehicular and pedestrians flows.

Minor street left turns must deal not only with all major street traffic flow but with two pedestrian’s flows

and the opposing Minor Street through and right turn

movements.

Through this movements the conflict volume (Vcx) for the

given movement ―x‖ is can be computed.

As an example the formula of conflict volume for movement

7 for three legged intersection shown in Fig. computed as:

Vc7 = 2Vc4 + Vc5 + +Vc2 + 0.5V 3 + V 13 + V 15

Potential Capacity

Capacity is defined as the maximum number of vehicles, passengers, or the like, per unit time, which can be

accommodated under given conditions with a reasonable expectation of occurrence. Potential capacity

describes the capacity of a minor stream under ideal conditions assuming that it is unimpeded by other

movements and has exclusive use of a separate lane.

Once of the conflicting volume, critical gap and follow up time are known for a given movement its potential

capacity can be estimated using gap acceptance models. The concept of potential capacity assumes that all

available gaps are used by the subject movement i.e. there are no higher priority vehicular or pedestrian

movements and waiting to use some of the gaps it also assumes that each movement operates out of an

exclusive lane. The potential capacity of can be computed using the formula:

)/fxtcx(ve

)/cxtcx(ve

cxvpxc3600

1

3600

Where,

Figure 24three legged intersection

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cpx = potential capacity of minor movement x (veh/h)

vcx = conflicting flow rate for movement x (veh/h)

tcx = critical gap for minor movement x

tf,x = follow-up time movement x.

Movement Capacity and Impedance Effects

Vehicles use gaps at a TWSC intersection in a prioritized manner. When traffic becomes congested in a high-

priority movement, it can impede lower-priority movements that are streams of Ranks 3 and 4 as shown in

Fig. 30:4 from using gaps in the traffic stream, reducing the potential capacity of these movements. The ideal

potential capacities must be adjusted to reflect the impedance effects of higher priority movements that may

utilize some of the gaps sought by lower priority movements. This impedance may come due to both

pedestrians and vehicular sources called movement capacity.

The movement capacity is found by multiplying the potential capacity by an adjustment factor. The

adjustment factor is the product of the probability that each impeding movement will be blocking a subject

vehicle. That is

∑

Where, Cmx = movement capacity, movement x,

veh/hr

Cpx = Potential capacity movement x, veh/hr

Pvi = probability that impeding vehicular

movement ―i‖ is not blocking the

subject flow; (also referred to as the

vehicular impedance factor for

movement ―i‖

Ppi = probability that impeding

pedestrian movement ―j‖ is not

blocking the subject flow; also referred to us the pedestrian impedance factor for the movement ―j‖

Vehicular Movements

Priority 2 vehicular movements LTs from major street and RTs from Minor Street are not impeded by any

other vehicular flow, as they represent the highest priority movements seeking gaps. They are impeded,

however, by Rank 1 pedestrian movements. Priority 3 vehicular movements are impeded by Priority 2

vehicular movements and Priority l and 2 pedestrian movements seeking to use the same gaps. Priority 4

vehicular movements are impeded by Priority 2 and 3 vehicular movements, and Priority 1 and 2 pedestrian

movements using the same gaps. Following Table lists the impeding flows for each subject movement in a

four leg.

Generally the rule stated the probability that impeding vehicular movement ―i‖ is not blocking the subject

movement is computed as

Table 4Relative pedestrian/vehicle hierarchy

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Where,

vi = demand flow for impeding movement ―i‖

Cmi=movement capacity for impeding movement ―i‖ veh/hr

Pedestrian impedance factors are computed as:

Pedestrian Movements

One of the impeding effects for all the movement is pedestrian’s movement. Both approaches of Minor-street

vehicle streams must yield to pedestrian streams. Table. Shows that relative hierarchy between pedestrian and

vehicular streams used. A factor accounting for pedestrian blockage is computed by following Eqn. on the

basis of pedestrian volume, the pedestrian walking speed, and the lane width that is:

(

⁄ )

Where, Pedestrian impedance factor for impeding pedestrian movement ―j‖

= pedestrian flow rate, impeding movement ―j‖, peds/hr

w = lane width, m

= pedestrian walking speed m/s

Determining Shared Lane Capacity

The capacities of individual streams (left turn, through and right turn) are calculated separately. If the streams

share a common traffic lane, the capacity of the shared lane is then calculated according to the shared lane

procedure. But movement capacities still represent an assumption that each minor street movement operates

out of an exclusive lane. Where two or three movements share a lane its combined capacity computed as:

∑∑

∑ (

)

Where, = shared lane capacity, veh/hr

= flow rate, movement ―y‖ sharing lane with other minor street flow

= movement capacity of movement ―y‖ sharing lane with other minor street

Determining Control Delay

Delay is a complex measure and depends on a number of variables it is a measure of driver discomfort,

frustration, fuel consumption, increased travel time etc. Total delay is the difference between the travel time

actually experienced and the reference travel time that would result during base conditions, in the absence of

incident, control, traffic, or geometric delay. Also, Average control delay for any particular minor movement

is a function of the Capacity of the approach and. The degree of saturation. The control delay per vehicle for a

movement in a separate lane is given by:

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5450

))(3600

(2)1()1(900

3600

T

mxc

xv

mxc

mxc

xv

mxc

xvT

mxcxd

Where,

dx = average control delay per vehicle for movement x, s/veh

Cmx = capacity of movement or shared lane x, veh/hr

T = analysis period, h (15min=0.25h)

Vx = demand flow rate, movement or shared lane x, veh/hr

Performance Measures

Four measures are used to describe the performance of TWSC

intersections: control delay, delay to Major Street through

vehicles, queue length, and v/c ratio. The primary measure that

is used to provide an estimate of LOS is control delay. This

measure can be estimated for any movement on the minor

(i.e., the stop-controlled) street. By summing delay estimates

for individual movements, a delay estimate for each minor street movement and minor street approach can be

achieved.

For AWSC intersections, the average control delay (in seconds per vehicle) is used as the primary measure of

performance. Control delay is the increased time of travel for a vehicle approaching and passing through an

AWSC intersection, compared with a free flow vehicle if it were not required to slow or stop at the

intersection. According to the performance measure of the TWSC intersection, LOS of the minor-street left

turn operates at level of service C approaches to B.

Example 5.1: four vehicles 6, 6.5, 6.75 and 6.9 m long, are distributed over a length of freeway lane 200 m.

long. What is the lane occupancy and density?

Example 5.2: In example 3.2 compute the average headways, spacing, clearances and gaps when the flow is

maximum.

Example 5.3: For the given three legged intersection of above figure the total volume pedestrian and

vehicular at each movement is given in the fig itself. Taking the following:

• The speed of the pedestrians 1.2m/s

• All flows contains 10% trucks

• The percentage of the grade is 0.00

• Ignore moments coming from south

bound

• The analysis period is 15min.(T=0.25)

Determine: The control delay and level of

service for movement 7?

Figure 25Three legged intersection

Table 5Level of service criteria for TWSC intersection

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Example 5.4: In the adjoining figure the amount of volume for the pedestrians, volume vehicles and the width

of the lane in (m) at each movement are given. Taking the following:

• The speed of the pedestrians 1.2m/s

• All flows contains 8% trucks

• The percentage of the grade is 0.00

• Ignore moments coming from south bound

Determine:

1. The potential capacities for movement 7

2. The movement capacities for movement 7

3. The control delay and level of service for movement 7

Figure 26Four legged intersection

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CHAPTER SIX: HIGHWAY CAPACITY AND LEVEL OF SERVICE

6.1Basic definitions related to capacity

Traffic Capacity:

Capacity of a transport facility is defined as the maximum number of vehicles, passengers, or the like, per unit

time which can be accommodated under given conditions with a reasonable expectation of occurrence. The

Highway Capacity Manual (2010) defines the capacity as the maximum howdy rate at which persons or

vehicles can be reasonably expected to traverse a point or a uniform segment of a lane or roadway during a

given time period, under prevailing roadway, traffic and control conditions. Several observations can be made

from the above definition. Although capacity is the maximum howdy rate, in many situations the break 15

minute flow rate is expressed as the capacity. The above definition also contains the term ―reasonably

expected‖ to account for the variation in traffic and driving habit at various location. However, it can be

termed as a probabilistic measure. Further, analytical derivations are possible for getting the maximum flow

rate, seldom it is achieved in the field. However, capacity measures are often empirically derived. Capacity is

usually defined for a point or a uniform segment where operating conditions do not vary.

The capacity measure depends on these operating conditions. The first is the traffic conditions and the factors

that influence the capacity includes vehicle composition, turning, movements, etc. The second factor is the

roadway conditions and it includes geometrical characteristics such as lane width, shoulder width, horizontal

alignment, and vertical alignment. The third factor is the control conditions such as the traffic signal timings,

round-about characteristics. It is also to be noted that the above capacity definition holds good for a point or at

a section of the road having uniform control conditions. Another aspect of the above capacity definition is the

expression that the maximum flow rate which accounts for the worst 15 minutes traffic within the peak hour

traffic. Lastly the term reasonable expectancy indicates that the capacity measure is probabilistic and not an

analytically derived deterministic value. The capacity measure is probabilistic, for it accounts for the

unexplainable variation in traffic and diverse driving characteristics.

Types of capacity:

An important operation characteristic of any transport facility including the multi-lane highways is the

concept of capacity. Capacity may be defined as the maximum sustainable flow rate at which vehicles or

persons reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a

specified time period under given roadway, geometric, traffic, environmental, and control conditions; usually

expressed as vehicles per hour, passenger cars per hour, or persons per hour.

There are two types of capacity, possible capacity and practical capacity. Possible capacity is defined as the

maximum number of vehicles that can pass a point in one hour under prevailing roadway and traffic

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condition. Practical capacity on the other hand is the maximum number that can pass the point without

unreasonable delay restriction to the average driver’s freedom to pass other vehicles.

Basic Capacity or theoretical capacity or capacity under ideal conditions: Basic Capacity maximum

number of passenger cars that can pass a given point on a lane in one hour under the most nearly ideal

roadway and traffic conditions which can possibly be attained.

* In bad prevailing traffic condition traffic congestion possible capacity zero.

* In ideal prevailing traffic and roadway condition possible capacity basic capacity.

Zero<Possible capacity<Basic capacity

Possible and Basic capacity represent the extreme cases of roadway and traffic condition. They are not

adopted for design purpose.

Importance of traffic capacity:

Following are some application of highway capacity:

Design features, governed by the capacity are highway type, number of lanes required, width of lane,

intersection geometry etc.

To study adequacy or deficiency of highway network present traffic volume is compared with the

capacity of the existing facility.

Improvements and changes in geometric features, junctions, traffic control devices traffic

management measures can be planned effectively if capacity of facility is known.

Determination of theoretical maximum capacity:

Basic or theoretical capacity, S

VC

1000

Where, C = capacity of single lane, vehicle per hour

V = speed, km/h

S = average centre to centre spacing of vehicles (space headway), m.

The value of S, the headway distance is known from the actual observations or can be calculated from

considerations of perception time braking distance and length of vehicle.

m ;254

278.02

1]

3600

1000[

3600

1000 22

f

VVtL

gf

VtVLS

Where, S = Spacing of vehicles, m

L = Length of vehicles, m

V = Speed, km/h

f = frictional factor

t = perception reaction time in sec.

The maximum theoretical capacity of a traffic lane may be obtained if the minimum time headway ht is

known:

hourper vehicle3600

thC

Speed Capacity Relationship:

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Peak value of the maximum theoretical capacity is reached at an optimum speed. As speed is increased

further, the maximum capacity of the lane starts decreasing due to increase in headway

6.2 Factors affecting capacity and LOS

BASE CONDITIONS

Many of the procedures in this manual provide a formula or simple tabular or graphic presentations for a set of

specified standard conditions, which must be adjusted to account for prevailing conditions that do not match.

The standard conditions so defined are termed base conditions.

Base conditions assume good weather, good pavement conditions, and users familiar with the facility, and no

impediments to traffic flow

Base conditions for uninterrupted flow facilities include the following:

Lane widths of 3.6 m,

Clearance of 1.8 m between the edge of the travel lanes and the nearest obstructions or objects at the

roadside and in the median,

Free-flow speed of 100 kmph for multilane highways,

Only passenger cars in the traffic stream (no heavy vehicles).

Level terrain.

No no-passing zones on two-lane highways, and

No impediments to through traffic due to traffic control or turning vehicles.

Base conditions for intersection approaches include the following:

Lane widths of 3.6 m,

Level grade,

No curb parking on the approaches.

Only passenger cars in the traffic stream,

No local transit buses stopping in the travel lanes,

Intersection located in a non-central business district area. and

Figure 27General concept of LOS

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No pedestrians.

In most capacity’ analyses, prevailing conditions differ from the base conditions. And Computations of

capacity, service flow rate, and level of service must include adjustments. Prevailing conditions are generally

categorized as roadway, traffic, or control.

ROADWAY CONDITIONS

Roadway conditions include geometric and other elements. Iii some cases, these influence the capacity of a

road: in others, they can affect a performance measure such as speed. but not the capacity or maximum flow

rate of the facility. Roadway factors include the following:

Number of lanes.

The type of facility and its development environment,

Lane widths.

Shoulder widths and lateral clearances,

Design speed,

Horizontal and vertical alignments, and

Availability of exclusive turn lanes at intersections.

The horizontal and vertical alignment of a highway depend on the design speed and the topography of the land

on which it is constructed.

In general the severity of the terrain reduces capacity and service flow rates. This is significant for two-lane

rural highways, where the severity of terrain not only can affect the operating capabilities of individual

vehicles in the traffic stream, bitt also can restrict opportunities for passing slow—moving vehicles.

TRAFFIC CONDITIONS

Traffic conditions that influence capacities and service levels include vehicle type and lane or directional

distribution.

Vehicle Type

The entry of heavy vehicles—that is vehicles other than passenger cars (a category that includes small trucks

and vans) —into the traffic stream affects the number of vehicles that can be served. Heavy vehicles are

vehicles that have more than four tires touching the pavement. Trucks, buses, and recreational vehicles (RVs)

are the three groups of heavy vehicles addressed by the methods. Heavy vehicles adversely affect traffic in

two ways:

They are larger than passenger cars and occupy more roadway space: and

They have poorer operating capabilities than passenger cars, particularly with respect to

acceleration, deceleration, and the ability to maintain speed on upgrades.

Directional and Lane Distribution

In addition to the distribution of vehicle types, two other traffic characteristics affect capacity, service flow

rates and level of service: directional distribution and lane distribution. Directional distribution has a dramatic

impact on two—lane rural highway operation which achieves optimal conditions when the amount of traffic is

about the same in each direction. Capacity analysis for multilane highways focuses on a single direction of

flow. Nevertheless, each direction of the facility usually is designed to accommodate the peak flow rate in the

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peak direction. Typically, morning peak traffic occurs in one direction and evening peak traffic occurs in the

opposite direction. Lane distribution also is a factor on multilane facilities. Typically, the shoulder lane carries

less traffic than other lanes.

CONTROL CONDITIONS such as the traffic signal timings, round-about characteristics.

6.3 Level of Service concept

Highway Capacity and Level of Service

―Level of Service (LOS)‖ is the qualitative measure describing the operational conditions within a traffic

stream, and their perception by motorists or passengers. The following factors may be considered to evaluate

LOS:

Speed and travel time, including the operating speed and overall travel time;

Traffic interruption or restrictions, with due consideration to the number of stops per mile;

Driving comfort and convenience reflecting the roadway and traffic conditions;

Freedom to maneuver to maintain the desired operating speed;

Economy, with due consideration operating cost

of vehicle.

HCM has categorized LOS depending upon the travel speed

and v/c ratio.

Level of Service A: Free flow operation; free-flow speeds

prevail; vehicles completely unimpeded in their ability to

maneuver within the traffic stream; average spacing of 528

ft. The effects of incidents are local and minimum.

Level of Service B: Reasonably free flow; generally free flow speed; ability to maneuver within the traffic

stream slightly restricted; average spacing 330 ft.

Level of Service C: Provides flow with speeds still at or near free flow speed; freedom to maneuver within

the traffic stream noticeably restricted and lane changes require more care by driver; average spacing 220 ft.

Local deterioration due to incidents in substantial and queues may be expected behind any significant

blockage.

Level of Service D: Speed begins to decline slightly with increasing flow; density begins to increasing

somewhat quickly; freedom to maneuver is more limited. Average spacing 165 ft.

Level of Service E: Describes operation at capacity at its highest density values; operations are volatile and

virtually no useable gaps exist in the traffic stream; maneuverability is extremely limited; average spacing is

110 ft. Level of physical and psychological comfort afforded the driver is poor.

Level of Service F: forced flow at low speeds; describes breakdown in vehicular flow at points of recurring

congestion such as merge, weave or diverging locations.

Passenger Car Unit (PCU):

The basic consideration behind using the concept of PCU is that different types of vehicle offer different

degree of interference to other traffic and it is necessary to bring all types to a common unit.

Six level of LOS: A, B, C, D, E, and F

A : Free flow condition

B & C : Stable flow

D & E : Unstable flow

& F : forced flow

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6.3 Computation of capacity and level of service f or two lane highways, multi lane highways and

freeways.

Multilane Highways

Increasing traffic flow has forced engineers to increase the number of lanes of highways in order to provide

good maneuvering facilities to the users. The main objectives is to present the basics of multilane highway, its

operational characteristics, capacity and level of service (LOS) concepts. An important parameter in the

capacity and LOS analysis is the free flow speed.

A highway is a public road especially a major road connecting two or more destinations. A highway with at

least two lanes for the exclusive use of traffic in each direction, with no control or partial control of access,

but that may have periodic interruptions to flow at signalized intersections not closer than 3.0 km is called as

multilane highway. Multilane highways exist in a number of settings, from typical suburban communities

leading to central cities or along high volume rural corridors that connect two cities or important activities

generating a considerable number of daily trips.

Highway Classification

Although there are various ways of classification of highways; the most common one is based on the number

of lanes. Thus highways may be classified as:

• Two lane highways.

• Three lane highway, and

• Four or more lane highway

The procedure for computing practical capacity for the uninterrupted flow condition is as follows:

Select an operating speed which is acceptable for the class of highways the terrain and the driver.

Determine the appropriate capacity for ideal conditions from table shown below.

Determine the reduction factor for conditions which reduce capacity (such as width of road,

alignment, sight distance, heavy vehicle adjustment factor).

Multiply these factors by ideal capacity value obtained from step 2.

Determination of Level of Service

The determination of level of service for a multilane highway involves three steps:

Determination of free-flow speed

Determination of flow rate

Table 6 Free flow speed and capacity for freeway and

multilane highway. (Source: HCM, 2000)

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Determination of level of service

Determination of Free-flow speed

Free-flow speed is the theoretical speed of traffic density, when the density approaches zero. It is the speed at

which drivers feel comfortable travelling under the physical, environmental and traffic conditions existing on

an uncongested section of multilane highway. In practice, free-flow speed is determined by performing travel

time studies during periods of low-to-moderate flow conditions.

From various studies of the flow characteristics, base conditions for multilane highways are defined as

follows:

Lane widths are 3.6 m.

Lateral clearance is 1.8 m.

The total lateral clearance is 3.6 m, with minimum of 1.8 m on either sides of in the direction of

travel. The clearance distance is measured from the edge of the outer lane and is inclusive of the

shoulder. If lateral clearance is more than 1.8 m, then it is considered as equal to 1.8 m.

No direct access points along the highway.

Divided highway.

Only passenger cars in the traffic stream.

A free-flow speed of 90 km/h or more.

When field data are not available, the free-flow speed can be estimated indirectly as follows:

Figure 30Density-flow relationships on multilane highways

(HCM, 2000)

Figure 30Speed-flow relationship on multilane highways

(HCM, 2000)

Figure 30Speed-flow curves with LOS criteria for multilane highways (HCM,

2000)

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Where,

FFS is the estimated FFS (km/h),

BFFS= base FFS (km/h),

= adjustment for lane width, from Table (km/h),

= adjustment for lateral clearance, from Table (km/h),

= adjustment for median type, from Table (km/h), and

= adjustment for access points, from Table (km/h).

Determining Flow Rate

Two adjustments must be made to hourly volume counts or estimates to arrive at the equivalent passenger-car

flow rate used in LOS analyses. These adjustments are the PHF and the heavy-vehicle adjustment factor. The

number of lanes also is used so that the flow rate can be expressed on a per-lane basis. These adjustments are

applied in the following manner using the equation below.

Where,

= 15-min passenger-car equivalent flow rate (pc/h/ln), = the hourly volume (veh/h), = the peak-hour factor, N = number of lanes,

= heavy-vehicle adjustment factor, and = driver population factor.

Heavy-vehicle adjustment factor:

Table 7Adjustment for lane width (Source: HCM, 2000)

Table 8Adjustment for median type (Source: HCM, 2000)

Table 10 Adjustment for lateral clearance (Source: HCM, 2000) Table 9 Adjustment for Access-point density (Source:

HCM, 2000)

Table 11 Level of Service criteria for a typical free flow speed of 100 km/hr. (Source: HCM,

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Besides that, the presence of heavy vehicles in the traffic stream decreases the FFS because base conditions

allow a traffic stream of passenger cars only. Therefore, traffic volumes must be adjusted to reflect an

equivalent flow rate expressed in passenger cars per hour per lane (pc/h/ln). This is accomplished by applying

the heavy-vehicle factor . Once values for ET and ER have been determined, the adjustment factors for

heavy vehicles are applied as follows:

( ) ( )

Where,

= adjustment factor for heavy vehicles.

ET and ER = equivalents for trucks and buses and for recreational vehicles (RVs), respectively,

PT and = proportion of trucks and buses, and RVs, respectively, in the traffic stream (expressed as a

d

e

c

i

m

al fraction),

Driver Population Factor

The adjustment factor reflects the effect weekend recreational and perhaps even midday drivers have on the

facility. The values for f range from 0.85 to 1.00. Typically, the analyst should select 1.00, which reflects

weekday commuter traffic (i.e., users familiar with the highway) unless there is sufficient evidence that a

lesser value, reflecting more recreational or weekend traffic characteristics, should be applied. When greater

accuracy is needed comparative field studies of weekday and weekend traffic flow and speeds are

recommended.

Determination of Level of Service

The level of service on a multilane highway can be determined directly from Fig. or Table based on the free-

flow speed (FFS) and the service flow rate (vp) in pc/h/ln. The procedure as follows:

Define and segment the highway as appropriate. The following conditions help define the

segmenting of the highway,

Change in median treatment

Change in grade of 2% or more or a constant upgrade over 1220 m

Change in the number of travel lanes

The presence of a traffic signal

Table 12 Passenger-car equivalent on extended general highway segments (Source: HCM,

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A significant change in the density of access points

Different speed limits

The presence of bottleneck condition

In general, the minimum length of study section should be 760 m, and the limits should be no closer

than 0.4 km from a signalized intersection.

On the basis of the measured or estimated free-flow speed on a highway segment, an appropriate speed-

flow curve of the same as the typical curves is drawn.

3. Locate the point on the horizontal axis corresponding to the appropriate flow rate (vp) in pc/hr/ln and

draw a vertical line.

4. Read up the FFS curve identified in step 2 and determine the average travel speed at the point of

intersection.

5. Determine the level of service on the basis of density region in which this point is located.

Density of flow can be computed as

Where,

D is the density (pc/km/ln),

is the flow rate (pc/h/ln), and S is the average passenger-car travel speed (km/h).

The level of service can also be determined by comparing the computed density with the density ranges shown

in table given by HCM. To use the procedures for a design, a forecast of future traffic volumes has to be made

and the general geometric and traffic control conditions, such as speed limits, must be estimated. With these

data and a threshold level of service, an estimate of the number of lanes required for each direction of travel

can be determined.

Figure 31Speed Flow curve with LOS criteria

Table 13 LOS criteria for Multi-Lane Highway

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Two lane highway

Capacity

The capacity of a two-lane highway is 1,700 1jc/h for each direction of travel. The capacity is nearly

independent of the directional distribution of traffic on the facility, except that for extended lengths of two-

lane highway, the capacity will not exceed 3,200 pc/h for both directions of travel combined. For short lengths

of two-lane highway such as tunnels or bridges a capacity of 3,200 to 3,400 pc/li for both directions of travel

combined may be attained but cannot be expected for an extended length.

Level of Service

On Class I highways, efficient mobility ¡s paramount, and LOS is defined in terms of both percent time-spent-

following and average travel speed. On Class II highways, mobility is less critical, and LOS is defined only in

terms of percent time spent-following, without consideration of average travel speed. Drivers will tolerate

higher levels of percent time spent-following on a Class II facility than on a Class I facility, because Class II

facilities usually serve shorter trips and different trip purposes.

Table 14LOS criteria for two lane highway Class II

Table 15 LOS criteria for two lane highway Class I

Figure 32LOS criteria (Graphical) for two lane highway class I

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Two-Way Segments

The two-way segment methodology estimates measures of traffic operation along a section of highway, based

on terrain, geometric design, and traffic conditions. Terrain is classified as level or rolling. Mountainous

terrain is addressed in the operational analysis of specific upgrades and downgrades. The methodology

typically is applied to highway sections of at least 3 km. Traffic data needed to apply the two-way segment

methodology include the two-way hourly volume, a peak-hour factor (PHF), and the directional distribution of

traffic flow. The PHF may be computed from field data, or appropriate default values may be selected. Traffic

data also include the proportion of trucks and recreational vehicles (RVs) in the traffic stream. The operational

analysis of extended two-way segments for a two-lane highway involves several steps, described in the

following sections.

Estimating FFS

The FFS can be estimated indirectly if field data are not available. This is a greater challenge on two-lane

highways than on other types of uninterrupted-flow facilities because the FFS of a two-lane highway can

range from 70 to 110 km/h. To estimate FFS, the analyst must characterize the operating conditions of the

facility in terms of a base free-flow speed (BFFS) that reflects the character of traffic and the alignment of the

facility. Because of the broad range of speed conditions on two-lane highways and the importance of local and

regional factors that influence driver-desired speeds, no guidance on estimating the BFFS is provided.

Estimates of BFFS can be developed based on speed data and local knowledge of operating conditions on

similar facilities. The design speed and posted speed limit of the facility may be considered in determining the

BFFS; however, the design speeds and speed limits for many facilities are not based on current operating

conditions. Once BFFS is estimated, adjustments can be made for the influence of lane width, shoulder width,

and access-point density. The FFS is estimated using the following equation

Where = Adjustment for lane and shoulder width, from Table

Adjustment for access points, from Table

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Determining Demand Flow Rate

Three adjustments must be made to hourly demand volumes, whether based on traffic counts or estimates, to

arrive at the equivalent passenger-car flow rate used in LOS analysis. These adjustments are the PHF, the

grade adjustment factor, and the heavy vehicle adjustment factor. These adjustments are applied according to

equation:

Where

= passenger-car equivalent flow rate for peak 15-min period (pc/h),

V = demand volume for the full peak hour (veh/h),

PHF = peak-hour factor,

= grade adjustment factor, and

= heavy-vehicle adjustment factor.

Heavy-Vehicle Adjustment Factor

Table 18 Grade Adjustment factor (fg) to determine speeds on two-way and directional segments

Table 19Grade Adjustment factor (fg) to determine the percent time-spent-following on two-way and directional

segments

Table 16 Adjustment (fLs) for lane width and shoulder width

Table 17Adjustment (fA) for access-point density

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Once values for ET and ER have been determined, the adjustment factor for heavy vehicles is computed using

equation

( ) ( )

Where,

= adjustment factor for heavy vehicles. ET and ER = equivalents for trucks and buses and for recreational vehicles (RVs), respectively,

PT and = proportion of trucks and buses, and RVs, respectively, in the traffic stream (expressed as a decimal fraction),

Determining Average Travel Speed

The average travel speed is estimated from the FFS, the demand flow rate, and an adjustment factor for the

percentage of no-passing zones. Average travel speed is then estimated using equation

Where

Table 20 Passenger-car equivalents for trucks and RVs to determine speeds on two-way and directional segments

Table 21 Passenger-car equivalents for trucks and RVs to determine time-spent-following on two-way and directional segments

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ATS = average travel speed for both directions of travel combined (km/h.

= adjustment for percentage of no-passing zones and

= passenger-car equivalent flow rate for peak 15-min period (pc/h).

Determining Percent Time-Spent- Following

The percent time-spent-following is estimated from the demand flow rate, the directional distribution of

traffic, and the percentage of no-passing zones. Percent time-spent-following is then estimated using equation.

Where

PTSF = percent time-spent-following,

BTSF = base percent time-spent-following for both directions of travel combined, and

= adjustment for the combined effect of the directional distribution of traffic and of the

percentage of no-passing zones on percent time-spent following.

Appropriate values of base percent-time-spent following can be determined from equation

( )

Determining LOS

The first step in determining LOS is to compare the passenger-car equivalent flow rate (Vp)) to the two-way

capacity of 3,200 pc/h. If vp is greater than the capacity, then the roadway is oversaturated and the LOS is F.

Table 22 Adjustment factor (fnp) for effect of no-passing zones on average travel speed on two-way segments

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Similarly, if the demand flow rate in either direction of travel—as determined from the two-way flow rate and

the directional split is greater than 1 ,700 pc/h, then the roadway is oversaturated and the LOS is F. In LOS F,

percent time—spent—following is nearly 100 percent and speeds are highly variable and difficult to estimate.

When a segment of a Class I facility has a demand less than its capacity, the LOS is determined by locating a

point on figure 22 that corresponds to the estimated percent time-spent-following and average travel speed. If

a segment of a Class II facility has a demand less than its capacity, the LOS is determined by comparing the

percent time spent—following with the criteria in table 6. The analysis should include the LOS and the

estimated values of percent time—spent—following and average travel speed. Although average travel speed

is not considered in the LOS determination for a Class II highway, the estimate may be useful in evaluating

the quality of service of two—lane highway facilities, highway networks, or systems including the segment.

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Table 23 Adjustment factor (fnp) for combined effect of directional distribution of traffic and percentage of no-passing zones on percent

time-following on two-way segments

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Basic Freeway Segment

Freeway facilities are composed of connected segments consisting of basic freeway segments, ramp segments,

and weaving segments. When several of these segments occur in sequence they form a freeway facility. A

freeway facility is the fundamental unit of analysis. A freeway facility is analyzed by direction, and the

independent analysis of both directions constitutes the analysis of a two direction freeway facility.

Base Conditions for Basic Freeway Segments

The base conditions under which the full capacity of a basic freeway segment is achieved are good weather,

good visibility, and 110 incidents or accidents. For the analysis procedures in this chapter, these base

conditions are assumed to exist. If any of these conditions fails to exist, the speed, LOS, and capacity of the

freeway segment all tend to be reduced.

The specific speed-flow-density relationship of a basic freeway segment depends on prevailing traffic and

roadway conditions. A set of base conditions for basic freeway segments has been established. These

conditions serve as a starting point for the methodology in this chapter.

Minimum lane widths of 3.6 in;

Minimum right-shoulder lateral clearance between the edge of the travel lane and the nearest obstacle

or object that influences traffic behavior of 1.8 in;

Minimum median lateral clearance of 0.6 in;

Traffic stream composed entirely of passenger cars;

Five or more lanes for one direction (in urban areas only)

Interchange spacing at 3 km or greater;

Level terrain, with grades no greater than 2 percent; and

A driver population composed principally of regular users of the facility.

These base conditions represent a high operating level, with a free-flow speed (FFS) of 110 km/h or greater.

LOS

A basic freeway segment can be characterized by three performance measures: density in terms of passenger

cars per kilometer per lane, speed in terms of mean passenger-car speed, and volume-to-capacity (v/c) ratio.

Each of these measures is an indication of how well traffic flow is being accommodated by the freeway.

The measure used to provide an estimate of level of service is density. The three measures of speed, density,

and flow or volume are interrelated. If values for two of these measures are known, the third can be computed.

Figure 33Freeway and its components

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Level-of-service thresholds for a basic freeway segment are summarized below.

Table 25 required input data and default value for basic freeway segment

Table 26 Example service volumes for basic freeway segment (see footnote for assumed values)

Table 24 LOS criteria for basic freeway segment

Table 27LOS criteria for Basic freeway segments

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Dete

rmining FFS

Figure 34 Speed-flow curves and LOS for basic freeway segments

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FFS is the mean speed of passenger cars measured during low to moderate flows (up to 1300 pc/h/in). For a

specific segment of freeway, speeds are virtually constant in this range of flow rate. Two methods can be used

to determine the FFS of a basic freeway segment: field measurement and estimation with guidelines provided

the field-measurement procedure is provided for users who prefer to gather these data directly. However, field

measurements are not required for application of the method. If field-measured data are used no adjustments

are made to the free-flow speed. The speed study should be conducted at a location that is representative of

the segment when flows and densities are low (flow rates may be up to 1,300 pc/h/ln).

If field measurement of FFS is not possible. FFS can be estimated indirectly on the basis of the physical

characteristics of the freeway segment being studied. The physical characteristics include lane width, number

of lanes, right-shoulder lateral clearance, and interchange density. Following is used to estimate the free-flow

speed of a basic freeway segment:

Where, BFFS - base free-flow speed, 110 (urban) or 120 km/h (rural)

- Adjustment for lane width from Table 28, km/h

- Adjustment for right shoulder clearance from Table 29, km/h

- Adjustment for number of lanes from Table 30, km/h

- Adjustment for interchange density from Table 31, km/h

Determination of peak flow rate

Table 28 Adjustment for Lane Width

Table 29 Adjustments for right-shoulder Lateral Clearance

Table 30Adjustments for number of lanes

Table 31Adjustments for Intersection density snlkh

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The hourly flow rate must reflect the influence of heavy vehicles, the temporal variation of traffic flow over

an hour, and the characteristics of the driver population. These effects are reflected by adjusting hourly

volumes or estimates, typically reported in vehicles per hour (veh/h), to arrive at an equivalent passenger-car

flow rate in passenger cars per hour (pc/h). The equivalent passenger-car flow rate is calculated using the

heavy-vehicle and peak-hour adjustment factors and is reported on a per lane basis (pc/h/in). Following is

used to calculate the equivalent passenger-car flow rate.

Where

= passenger-car equivalent flow rate for peak 15-min period (pc/h),

V = demand volume for the full peak hour (veh/h),

PHF = peak-hour factor,

N = number of lanes

= driver population factor, and

Driver Population Factor

The adjustment factor reflects the effect weekend recreational and perhaps even midday drivers have on the

facility. The values for f range from 0.85 to 1.00. Typically, the analyst should select 1.00, which reflects

weekday commuter traffic (i.e., users familiar with the highway) unless there is sufficient evidence that a

lesser value, reflecting more recreational or weekend traffic characteristics, should be applied. When greater

accuracy is needed comparative field studies of weekday and weekend traffic flow and speeds are

recommended.

= heavy-vehicle adjustment factor.

( ) ( )

Where,

= adjustment factor for heavy vehicles. ET and ER = equivalents for trucks and buses and for recreational vehicles (RVs), respectively,

PT and = proportion of trucks and buses, and RVs, respectively, in the traffic stream (expressed as a decimal fraction),

The effect of heavy vehicles on traffic flow depends on grade conditions as well as traffic composition.

Passenger-car equivalents can be selected for one of three conditions: extended freeway segments, upgrades,

and down2rades

Table 32Passenger-car equivalents on extended freeway segments

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Table 33 Passenger-car equivalents for trucks and buses on upgrades

Table 34Passenger-car equivalents for RVs on Upgrade

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For RVs, downgrades may be treated as level terrain.

Determining LOS

The first step in determining LOS of a basic freeway segment is to define and segment the freeway facility as

appropriate. Second, on the basis of estimated or field measured FFS, an appropriate speed-flow curve of the

same shape as the typical curves Figure 34 is constructed. On the basis of the flow rate, and the constructed

speed-flow curve, an average passenger-car speed is read on the y-axis of figure 34. The next step is to

calculate density using following equation

Density of flow can be computed as

Where, D is the density (pc/km/ln),

is the flow rate (pc/h/ln), and S is the average passenger-car travel speed (km/h).

Examples:

Example 6.1: A segment of undivided four-lane highway on level terrain has field-measured FFS 74.0-km/h,

lane width 3.4-m, peak-hour volume 1,900-veh/h, 13 percent trucks and buses, 2 percent RVs, and 0.90 PHF.

What is the peak-hour LOS, speed, and density for the level terrain portion of the highway?

Example 6.2: A segment of an east-west five-lane highway with two travel lanes in each direction separated

by a two-way left-turn lane (TWLTL) on a level terrain has- 83.0-km/h 85th-percentile speed ,3.6-m lane

width, 1,500-veh/h peak-hour volume, 6 % trucks and buses, 8 access points/km (WB), 6 access points/km

(EB), 0.90 PHF, 3.6-m and greater lateral clearance for westbound and eastbound. What is the LOS of the

highway on level terrain during the peak hour?

Example 6.3: A Class I two-lane highway has a base free-flow speed of 100 km/h. Lane width is 3.6 m and

shoulder width is 1.2 m. There are six access points per kilometer. The roadway is located in rolling terrain

with 40 percent no-passing zones. The two-way traffic volume is 800 veh/h, with a PHF of 0.90. The

Table 35Passenger-car equivalents for trucks and buses on downgrade

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directional split is 60/40. Traffic includes 5 percent trucks and 10 percent recreational vehicles. Determine the

level of service.

Example 6.4: A Class I two-lane highway has a base free-flow speed of 100 km/h. Lane width is 3.4 m and

shoulder width is 1.2 m. There are 12 access points per kilometer. The roadway is located in rolling terrain

with 50 percent no-passing zones. The two-way traffic volume is 1600 veh/h, with a PHF of 0.90. The

directional split is 50/50. Traffic includes 14 percent trucks and 4 percent recreational vehicles. Determine the

level of service.

Example 6.5: The Highway A Class Il two-lane highway segment on a scenic and recreational route. What is

the two-way segment LOS?

The Facts:

Example 6.6: The Freeway Existing four-lane freeway, rural area, very restricted geometry, rolling terrain,

110-km/h speed limit. What is the LOS during the peak hour?

The Facts

Example 6.7: New suburban freeway is being designed. How many lanes are needed to provide LOS D

during the peak hour?

70/30 directional split,

7 percent RV5,

90-km/h base FFS,

3.0-m lane width,

10-km roadway length, and

6 access points/km.

1,050 veh/h (two-way volume),

5 percent trucks and buses,

0.85 PHF,

Rolling terrain,

0.6-m shoulder width,

60 percent no-passing zones,

Two lanes in each direction,

3.3-m lane width,

0.6-m lateral clearance,

Commuter traffic,

2,000-veh/h peak-hour volume (one direction),

5 percent trucks,

0.92 PHF,

0.6 interchanges per kilometer,

Rolling terrain.

3.6-m lane width,

1.8-m lateral clearance,

15 percent truck

4,000-veh/h peak-hour volume (one direction),

3 percent RVs,

0.85 PHF,

0.9 interchanges per kilometer,

Level terrain.

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CHAPTER SEVEN: PARKING STUDIES AND ANALYSIS

7.1Types of parking faci l i ties - on street parking and off street Parking faci l i t ies

Parking problem is the one of the serious problems for planners and traffic engineer.

Effects

Congestion: Capacity of the street is reduced

Journey speed drops down

Journey time, delay increases

Accidents: Careless opening of the door of vehicles

Bringing a car to the parking location from the mainstream

Moving out of a parked position

Obstruction to the emergency services

Fire fighting vehicles; block access to building

Environment

Methods of parking surveys

Parking space inventory: Sketch plan is prepared; number and type of parking space are counted and recorded.

Questionnaire type parking usage survey (to know parking demand)

Cordon count

Purpose of parking studies:

To determine the congestion in the city or town areas

To assess the suppressed parking demand

To estimate the desires and demands of the public for parking facility

To decide the capacity, location and type of future parking facilities

Method of Parking

On street parking

Off street parking

On street parking

Vehicles are parked on the sides of the street itself.

This will be usually controlled by government agencies itself. Common types of on-street parking are

as listed below.

This classification is based on the angle in which the vehicles are parked with respect to the road

alignment.

As per IRC the standard dimensions of a car is taken as 5mX2.5m.

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Parallel parking:

Angle parking

30 parking:

45 parking:

60 parking

Right angle parking:

Figure 35 on street parking

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Off Street Parking

In many urban centers, some areas are exclusively

allotted for parking which will be at some distance

away from the main stream of traffic. Such a parking is

referred to as off-street parking. They may be operated

by either public agencies or private firms. A typical

layout of an off-street parking is shown in figure.

TYPES

Surface parking

Multi-storey car parks

Roof car parks

Mechanical car parks

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Underground car parks

7.2Parking studies and analysis

PARKING STATISTICS:

Parking accumulation: The total

number of vehicles parked in an area at a

specific moment. The curve of parking

accumulation for typical day is given in fig.

Figure 36 off street parking

12 3 6 129 3 6 9 12

Average Accumulation

Time of day

Nu

mb

er

of ve

hic

les p

ark

ed

Figure 37Parking accumulation diagram

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../supporting files/gallary'/videos/mechanical parking system PSH - YouTube.FLV

../supporting files/gallary'/videos/Most Advanced Automated Garage In The World - YouTube.FLV


Parking volume: The number of vehicles parking in a particular area over a given period of time. It is usually

measured in vehicles per day.

Parking load: The area under the parking accumulation curve during a specific period. For example the

hatched area (in fig.) represents the parking load in vehicle-hour for a period of 3 hours from 6am to 9 am.

Parking duration: The length of time spent in a parking space.

Parking index: percentage of parking bays actually occupied by parked vehicles as compared to the

theoretical numbers available.

Parking turnover: Rate of the usage of the available parking space. Thus if there were 10 parking spaces

used by 100 vehicles in a period of, say 12 hours, then the parking turnover would be

Types of parking surveys

Parking space inventory

The first step in a parking survey is to collect data on the amount, type and location of space actually or

potentially available for parking in an area. The area to be surveyed should first be delineated. The CBD is

usually the area where parking survey is needed. Items to be recorded are:

Total length of curb, and lengths governed by no waiting and limited waiting restrictions.

Number of parking space provided in the street

Street width

Location of bus stops, Bus bays, pedestrian crossings, loading zones, taxi stands and other features

that are likely to affect the use of the street for parking.

Traffic management measures in force, such as prohibited turns, one way streets etc.

Number and type of traffic signs for regulation of parking

Private streets

Vacant or unused land suitable for temporary or permanent parking space

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A typical sketch plan for a street with the above information recorded given in fig.

Parking usage by patrol

Purpose: the purpose of parking usage survey is to obtain data on the extent of usage of parking spaces. The

survey will include counts of parked vehicles at regular intervals through a period, covering both the morning

and evening peak period, and the parking accumulation and turn over.

The survey can be for on-street and off-street parking. The general methodology for both the survey is similar.

This method consists of making periodic observations of

parked vehicle on each patrol.

Mapping the street system: The first step is to prepare

a map of the street system that will be covered by the

patrol, showing therein its sub-division into sections.

Street junctions make convenient points for determining

the sections.

The recording can be for both sides of the road or

separately for each side. The map and the forms should

clearly show the direction of travel by the patrol man

and the side or sides where observations are to be

recorded (by arrows). The length of streets to be covered

by a patrol is limited by:

The speed of walking while noting the registration numbers and

The frequency of patrol

A speed of 900m in half an hour is useful guide, which should be altered to suit the density of

parking.

Frequency of patrol:

A frequency of half an hour is considered to be satisfactory for on-street parking, while a frequency of one

hour could be used for off-street parking. A frequency of half an hour is likely to miss short term parkers (up

to 29 minutes duration) and this makes it necessary to have more frequent patrols in selected areas where short

term parking may be significant e.g. near banks, post offices etc.

Method of observation:

Figure 38Parking space inventory

Figure 39Patrol map

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Usually patrols are by foot, but where vehicles are not parked, too close to one another a moving car may also

be used. As an aid a tape recorder may be used to record the registration numbers of vehicles.

Timing of survey:

The survey should be done on a typical week day, free from factors likely to result in non-representative

characteristics. The period of the survey is usually 10 to 12 hours, so as to cover the arrival and departure of

commuters and shoppers.

Equipment and form of recording

Each observer will be equipped with a watch, a pencil, a supply of forms, a map of the street and a board. A

specimen form is shown in fig.

Parking Survey Patrol : 2

On s treet

Street : King Street

Section : AB

Side : Right

Date : Friday, 02/02/2011

AM

PM

8:00 8:30 9:00 9:30 10:00 10:30 11:00 11:30 12:00 12:30 1:00 1:30

L R L R L R L R L R L R L R L R L R L R L R L R

421 421 421 336 814 836 129 129 129 129 451 451

545 356 353 402 336 402 402 402 402 402 402 402

817 27 350 421 402 921 921 921 921 921 921 921

98 114 333 921 421 56 56 56 56 56 56

113 27 350 421 455 455 455 455 455 455 455

545 714 114 333 333 312 312 312 312 312 312

120 98 27 356 356 356 356 356 356 444 444

817 113 97 114 114 99 99 99 99 99 99

656 714 27 97 97 97 97 97 97 221

545 98 97 221 221 221 221 221 221 76

212 113 221 65 76 76 76 76 76 714

149 450 714 714 714 714 714 714 714 98

120 656 98 95 98 98 98 98 98 113

88 545 113 113 113 113 113 113 113 77

817 212 450 450 450 450 450 79 77 426

44 149 428 426 426 426 426 426 426 21

150 656 656 345 345 345 21 21 118

120 118 118 118 118 118 118 118 244

817 212 212 212 212 212 212 244 150

44 149 150 150 150 150 150 150 817

929 150 149 136 136 136 817 817

817 817 817 817 817 929 929

929 929 929 929 929

444 444 444 444

Questionnaire type parking usage survey

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The questionnaire type parking usage survey involves interviews with the drivers who use the parking

facilities.

The survey can either be made by making enquiries among the car owners living in the vicinity of the survey

area, or by making enquiries among the drivers of cars seen to park in the area at the time of survey.

In this interview of actual parkers, the information collected should include:

Address of origin of the trip

Address of destination of the trip

Trip purpose

Time of arrival at the parked place

Time of departure from the parked place

Type of parking space used

Type of vehicle

Normally one interviewer is required to cover about fifteen spaces. All the parkers in 8 or 10 hours period are

interviewed. The duration of the survey may be a single day or may be spread over a number of days.

Cordon count

In this method, the area to be surveyed is demarcated by a cordon line which is crossed by the roads

emanating from the area. Counting stations are established at these crossing points and a count is made of all

the vehicles entering and leaving the area. The difference between the top traffic gives the number of vehicles

parked or in motion in the area.

Examples:

Example 7.1: From an in-out survey conducted for a parking area consisting of 40 bays, the initial count was

found to be 25. Table gives the result of the survey. The number of vehicles coming in and out of the parking

lot for a time interval of 5 minutes is as shown in the table 1. Find the accumulation, total parking load,

average occupancy and efficiency of the parking lot.

Table 1:

In-out survey

data

Time 5 10 15 20 25 30 35 40 45 50 55 60

In 3 2 4 5 7 8 2 4 6 4 3 2

Out 2 4 2 4 3 2 7 2 4 1 3 5

Example 7.2: The parking survey data collected from a

parking lot by license plate method is s shown in the table

2 below. Find the average occupancy, average turn-over,

parking load, parking capacity and efficiency of the parking

lot.

Table 2 : License plate parking survey data

Bay Time

0-15 15-30 30-45 45-60 1 1456 9813 - 5678

2 1945 1945 1945 1945 3 3473 5463 5463 5463

4 3741 3741 9758 4825 5 1884 1884 - 7594

6 - 7357 - 7893 7 - 4895 4895 4895

8 8932 8932 8932 - 9 7653 7653 8998 4821

10 7321 - 2789 2789 11 1213 1213 3212 4778

12 5678 6678 7778 8888

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CHAPTER EIGHT: TRAFFIC SAFETY

8.1Accident studies and analysis

The problem of road accident is ever-existing and acute in highway transportation due to complex flow of

vehicular traffic, presence of mixed traffic and pedestrians. Consequence of traffic accident may include

property damages, personal injuries, fatal cases as well as social and moral effect of a community. Systematic

studies to investigate the causes of accidents and preventive measures in terms of designs and control may

decrease the rate of accidents. The cost of the traffic accident helps to evaluate an improvement scheme for

reducing accidents.

Traffic safety is most important issue in the present context of motorization. More than 1.3 million people are

killed in the world only by the road accidents each year and 50 million people injured and disabled as the

result. This is number one cause of death for young people worldwide. In Nepal there were 1734 deaths due to

road accident 2009/2010 and 4230 were seriously injured.

Definitions:

Road accident: An accident (collision, overturning or slipping) which occurred or originated on a road open

to public traffic resulting in either injury or loss of life, or damage to property, in which at least one moving

vehicle was involved.

Person killed: (given in the Convention of Road Traffic (Vienna, 1968) ―Any person who was killed outright

or who died within 30 days as a result of the accident.‖

Fatal accident: an accident in which one or more person were killed.

International comparison of road accidents:

It is important to compare the road safety situations in different countries. The data vary in different countries

due to economic conditions, education and literacy, climate, types of vehicles, traffic conditions, population

and vehicle density, and others. Most countries maintain records and statistics about road safety and accidents.

There are three methods to compare the road safety conditions:

country Road death, per

100,000 population

Total population,

million

Total number

killed in road death

Italy

Japan

Korea

Luxembourg

Netherlands

New Zealand

Norway

Poland

Portugal

9.7

6.7

13.6

11.1

4.9

10.7

5.7

15.0

12.3

57.9

127.7

48.1

0.5

16.3

4.1

4.6

38.2

10.5

5 625

8 492

6 563

50

804

436

259

5 712

1 294

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a) The number of road

deaths for every

100,000 population is a

measure of the public

health risk associated with road trauma. (Data for 2004 source: Australian transport safety Bureau)

b) The number of deaths for every 10,000 registered vehicles is a means of comparing road death levels among nations by taking into account their different levels of motorization.

c) The number of road deaths for every 100 million vehicle kilometers traveled is a direct measure of the risk associated with road travel.

Country Road death per 100,000

Vehicle-kilometer traveled

Total vehicle kilometer

traveled (100 million)

Total road death

Finland

France

Germany

0.7

1.0

0.8

509

5600

6971

375

5530

5842

Objectives of accident study:

The main objectives of accident studies and recording are to work out solutions after proper analysis and

study. They can be listed as:

To study the causes of accidents and to suggest corrective treatment at potential locations.

To evaluate performance of existing facilities in terms of safety

U K

U S A

5.6

14.5

59.8

293.7

3 368

42 636

country Road death per 10,000

Registered vehicle

Total registered

vehicles, million

Total number killed

in road death

Spain

Sweden

Switzerland

United Kingdom

1.8

0.9

1.0

1.0

26.4

5.1

5.0

33.0

4741

480

510

3368

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To support proposed design.

To carry out before and after studies of improvement schemes.

To make computations of financial loss during accidents.

To give economic justification for improvement schemes.

To define and identify high-accident locations.

8.2Causes of acc idents - The Road, The vehicle, the road user and the Environment

Smeed and Jeffcoate have presented a relationship between the number of motor vehicles (N) per population

(P) and the number of deaths (D) per vehicle population (N).

3/2

0003.0

P

N

N

D

A research on accident has established the major causes and their contribution to the traffic accident as in the

table below:

Prime causes of Road accidents Percentage of accidents

Human factors alone

Human + Road

Human + Vehicle

Road factors alone

Vehicle factors alone

Human + Road + Vehicle

Total

65

25

5

2

2

1

100

A road accident may be caused due to combination of several reasons

Different causes:

Drivers: Excessive speed, carelessness, violation of rules and regulations, fatigue, sleep, alcoholic etc.

Figure 40 Causes of accident 2069/70 and 2070/71

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Pedestrians: Violating regulations, carelessness.

Passengers: Alighting from or getting into moving vehicles.

Vehicle defects: Failure of brakes, steering system, lighting system, tyre brust, etc.

Road condition: Slippery or skidding road surface, pot holes and other damaged conditions.

Road design: Defective geometric design.

Weather: Fog, snow, rainfall, dust, smoke etc which restrict the normal visibility.

Animals: Stray domestic animal and wild animals.

Other causes: Incorrect signs or signals, advertisement boards, service stations etc.

8.3 Accident studies, records and analysis

Various steps involved in accident studies are: a) Collection of accident data: Accident data are collected in a standard form. Details to be collected are:

i. General: Date, time, persons involved in the accident, classification of accident (fatal, serious,

minor etc.)

ii. Location: Description and details of the location site of accident.

iii. Details of vehicles involved: Registration number, description of vehicle, loading details, vehicles

details, vehicular defects etc.

iv. Nature of accident: Condition of vehicles, details of collisions, pedestrians or objects involved

damages etc.

v. Road and traffic conditions: Details of road geometry, surface characteristics, traffic condition

(density etc.)

vi. Primary causes of accident: Various possible cause and primary causes.

vii. Accident costs: Property damages, personal injuries and causalities.

b) Accident statistics

Types of accident statistics:

Accident statistics generally address and describe one of three principal informational elements:

Accident occurrence: Accident occurrence relates to the numbers and types of accidents that

occur, which are often described in terms of rates based on population or vehicle-miles

traveled.

Accident involvements: Accident involvement concerns the numbers and types of vehicles

and drivers involved in accidents with population-based rates a very popular method of

expression.

Accident severity: Accident severity is generally dealt with by proxy; the number of fatalities

and fatality rates are often used as a measure of the seriousness of accidents.

Accident Rates

Simple statistics citing total numbers of accidents, involvements, injuries, and/or deaths can be quite

misleading, as they ignore the base from which they arise. An increase in the number of highway fatalities in a

specific jurisdiction from one year to next must be matched against population and vehicle-usage patterns to

make any sense. For this reason, many accident statistics are presented in the form of rates.

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Accident rates generally fall into one of two broad categories: population-based rates, and exposure-based

rates.

Population-based accident rates

Some common bases for population-based rates include:

Area population

Number of registered vehicle

Number of licensed drivers

Highway mileage

These values are relatively static (they do not change radically over short periods of time) and do not depend

upon vehicle usage or the total amount of travel. They are useful in quantifying overall risk to individuals on a

comparative basis. Number of registered vehicles and licensed drivers may also partially reflect usage.

Population-based rates are stated according to;

Fatalities, accidents, or involvements per 100,000 area population

Fatalities, accidents, or involvements per 10,000 registered vehicles

Fatalities, accidents, or involvements per 10,000 licensed drivers

Fatalities, accidents, or involvements per 1,000 miles of highway

Exposure-based accident rates

Exposure based accident rates attempt to measure the amount of travel as a substitute for the individual’s

exposure to potential accident situations. The two most common bases for exposure-based rates are:

Vehicle miles traveled

Vehicle hours traveled

Exposure-based rates are stated according to:

Fatalities, accidents or involvements per 100,000,000 vehicle- miles traveled.

Fatalities, accidents or involvements per 10,000,000 vehicle-hours traveled.

Fatalities, accidents or involvements per 1,000,000 entering vehicles (for intersections only)

Severity Index:

A widely used statistic for the description of relative accident severity is the severity index (SI), defined as the

number of fatalities per accident .For the data of the previous example, there were 75 fatalities in a total of

2,360 accidents. This yields a severity index of:

Identifying high-accident locations

A Primary function of an accident record system is to regularly identify locations with an unusually high rate

of accidents and/or fatalities. Accident spot maps are a tool that can be used to assist in this task.

Figure shows a sample accident spot map. Coded pins or markers are placed on a map. Color or shape codes

are used to indicate the category and/or severity of the accident.

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“Before and after” accident analysis

When an accident problem has been identified, and an improvement implemented, the engineer must

evaluate whether or not the remediation has been effective in reducing the number of accidents

and/or fatalities.

A before and after analysis must be conducted. The length of time considered before and after the

improvement must be long enough to observe changes in accident occurrence. For most locations,

periods ranging from three months to one year are used. The length of the ―before‖ period and the

―after‖ period must be the same

The normal approximation test is often used to make this determination. The statistics Z is computed as:

√

) )

Test statistic representing the reduction in accident on the standard normal distribution

The normal distribution table is entered with this value to find the probability of Z being equal to or less than

Z1. If probability [Z ≤Z1] ≥ 0.95, the observed reduction in accidents is statically significant.

c) Site Analysis

One of the most important tasks in traffic safety is the study and analysis of site-specific accident information

to identify contributing causes and develop site remediation measures that will lead to improved safety.

Once location has been statistically identified as ―high-accident‖ location, detailed information is required in

two principal ways:

Occurrence of accident

Environmental and physical conditions

The best information on the occurrence of accidents is compiled by reviewing all accident reports for a given

location over a specified study period. This can be done using accident record. Environmental and physical

conditions are established by a through site investigation. Two primary graphical outputs are then prepared.

Figure 41Typical Accident spot map

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Collision diagram

Condition diagram

Collision diagram:

A collision diagram is a schematic representation of all accidents occurring at a given location over a specified

period. Depending upon the accident frequency, the ―specific period‖ usually ranges from one to three years.

Each collision is represented by a set of arrows, one for each vehicle involved, which schematically represents

the type of accident and directions of all vehicles. Arrows are generally labeled with codes indicating vehicle

types, date and time of accident, and weather conditions.

Condition diagram

A condition diagram describes all physical and

environmental conditions at the accident site. The

diagram must show all geometric features of the

site, the location and description of all control

devices (signs, signals, markings, lighting, etc.) and

all relevant features of the road side environment,

such as the location of carriage ways, roadside

objects, lane uses etc.

Figure 42 Condition Diagram

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Figure 43Collision Diagram

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8.3Engineering, Enforcement and Education measures for the prevention of accidents.

Measures for the reduction in accident rates

The various measures to decrease the accident rates may be divided into three groups:

Engineering

Enforcement ―3-Es‖

Education

Engineering measures

Road design: The geometric design features of the road such as sight distances, width of pavement, horizontal

and vertical alignment and intersection design elements are checked and corrected if necessary. The pavement

surface characteristics are checked and suitable maintenance steps taken to bring them upto the design

standards.

Preventive maintenance of vehicle: The braking system, steering and lighting arrangements on vehicle may be

checked.

Before and after studies: After making the necessary improvement in design and enforcing regulation, it is

again collect and maintain the record of accidents ―before and after‖ the introduction of preventive measures

to study their efficiency.

Road lighting: Lighting is particularly desirable at intersections, bridge sites and at places where there are

restrictions to traffic movements.

Enforcement measures

Speed control: Checks on spot speed of all fast moving vehicles should be done at selected locations and

timings and legal actions on those who violate the speed limits should be taken

Traffic control devices: Signals may be re-designed or signal system be introduce if necessary. Proper traffic

control device like signs, markings or channelizing island may be installed if necessary.

Training and supervision: the transport authorities should be strict in testing and issuing license to driver.

Medical check: The drivers should be tested for vision and reaction time at prescribed intervals.

Special precautions for commercial vehicles: having attendant to help and give proper direction to drivers of

heavy vehicles.

Observance of law and regulations: Traffic authorities should send study groups of trained personal, to

different locations to check whether the traffic regulations are being followed by the road users and also to

enforce the essential regulations.

Educational measures

Education of road users: The passengers and pedestrians should be taught the rules of the road, correct manner

of crossing etc.

Safety drive: Imposing traffic safety week when the road users are properly directed by the help of traffic

police and transport staff is a common means of training the public these days.

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Figure 44 Traffic Police Accident report

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Figure 45Traffic Police Accident report

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Example 8.1: The following are sample gross accident for a relatively small urban jurisdiction in the year 2003: Compute accident and fatality rates

Fatalities: 75

Fatal accidents: 60

Injury accidents: 300

PDO: 2000

Total involvements: 4100

Vehicle-miles travelled: 1,500,000,000

Registered vehicles: 100,000

Licensed drivers: 150,000

Area population: 300,000

Example 8.2

A signal is installed at a high-accident location to reduce the number of right-angle accidents that are

occurring. In the 6 months period prior to installing the signal, 10 such accidents occurred. In the 6 months

period following the installation of the signal, 6 such accidents occurred. Was this reduction statically

significant?

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CHAPTER NINE: TRAFFIC CONTROL AND REGULATION

9.1Warrants for traffic control signal system

Each and every intersection is not feasible for signalization. Traffic control signals should not be installed

unless one or more of the following signal warrants met.

IRC recommended:

Warranty I: Minimum vehicular volume

Min. 650 veh/hr on a major street in a single lane

800 veh/hr on two or more lanes

Min 200 veh/hr on a minor street in a single lane

250 veh/hr on a two or more lanes

But when 85th

percentile speed = 60kmph or when it is within built up areas, only 70% of the above warranty

is needed.

Warranty II: Interruption of continuous traffic

Interruption of continuous traffic on Major Street with 1000-1200veh/hr and there is delay or hazard

to traffic on minor street with a traffic of 100-150 veh/hr.

Warranty III: Minimum pedestrian volume

If on the major street, 600 veh/hr or more enter the intersection (both approaches) OR where there is a

raised median island 1.2m or more in width, 1000veh/hr or more (both direction) enter the

intersection;

In the above intersection if 150 or more pedestrians cross the major street & If 85th percentile speed exceeds

60kmph, 70% of above is sufficient.

Warranty IV: Accident experience

If the 5 or more reported accidents involving personal injury or property damage (IRs. 2000 or more);

Adequate trial of less restrictive remedies with satisfactory observance and enforcement have failed to reduce the accident rate;

The signal installation will not seriously disrupt traffic flow.

9.2 Design Pr inciples of Traffic Signal

The conflicts arising from movements of traffic in different directions is solved by time sharing of the

principle. The advantages of traffic signal includes an orderly movement of traffic, an increased capacity of

the intersection and requires only simple geometric design. However the disadvantages of the signalized

intersection are it affects larger stopped delays, and the design requires complex considerations. Although the

overall delay may be lesser than a rotary for a high volume, a user is more concerned about the stopped delay.

Definitions and notations

A number of definitions and notations need to be understood in signal design. They are discussed below:

Cycle: A signal cycle is one complete rotation through all of the indications provided.

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Cycle length: Cycle length is the time in seconds that it takes a signal to complete one full cycle of

indications. It indicates the time interval between the starting of green for one approach till the next time the

green starts. It is denoted by C.

Interval: Thus it indicates the change from one stage to another. There are two types of intervals - change

interval and clearance interval. Change interval is also called the yellow time indicates the interval between

the green and red signal indications for an approach. Clearance interval is also called all red is included after

each yellow interval indicating a period during which all signal faces show red and is used for clearing off the

vehicles in the intersection.

Green interval: It is the green indication for a particular movement or set of movements and is denoted by Gi.

This is the actual duration the green light of a traffic signal is turned on.

Red interval: It is the red indication for a particular movement or set of movements and is denoted by Ri.

This is the actual duration the red light of a traffic signal is turned on.

Phase: A phase is the green interval plus the change and clearance intervals that follow it. Thus, during green

interval, non-conflicting movements are assigned into each phase. It allows a set of movements to flow and

safely halt the flow before the phase of another set of movements start.

Lost time: It indicates the time during which the intersection is not effectively utilized for any movement. For

example, when the signal for an approach turns from red to green, the driver of the vehicle which is in the

front of the queue, will take some time to perceive the signal (usually called as reaction time) and some time

will be lost here before he moves.

The signal design procedure involves six major steps. They include the (1) phase design, (2) determination of

amber time and clearance time, (3) determination of cycle length, (4) apportioning of green time, (5)

pedestrian crossing requirements, and (6) the performance evaluation of the above design.

Phase design

The objective of phase design is to separate the conflicting movements in an intersection into various phases,

so that movements in a phase should have no conflicts. If all the movements are to be separated with no

conflicts, then a large number of phases are required. In such a situation, the objective is to design phases with

minimum conflicts or with less severe conflicts.

Figure 46Typical flow rates at a signalized movement snlkh

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There is no precise methodology for the design of phases. This is often guided by the geometry of the

intersection, flow pattern especially the turning movements, the relative magnitudes of flow. Therefore, a trial

and error procedure is often adopted. However, phase design is very important because it affects the further

design steps. Further, it is easier to change the cycle time and green time when flow pattern changes, where as

a drastic change in the flow pattern may cause considerable confusion to the drivers. Left turns is ignored. The

first issue is to decide how many phases are required. It is possible to have two, three, four or even more

number of phases.

Interval design

There are two intervals, namely the change interval and clearance interval, normally provided in a traffic

signal. The change interval or yellow time is provided after green time for movement. The purpose is to warn

a driver approaching the intersection during the end of a green time about the coming of a red signal. They

normally have a value of 3 to 6 seconds. The design consideration is that a driver approaching the intersection

with design speed should be able to stop at the stop line of the intersection before the start of red time.

Institute of transportation engineers (ITE) has recommended a methodology for computing the appropriate

length of change interval which is as follows:

Where y = Clearance interval

= 85th

percentile speed of approaching vehicles in m/s

g = grade of approach road

Figure 48Four legged intersection Figure 47Two Phase Signal

Figure 49Three possible ways for four phase signal

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The clearance interval is provided after yellow interval and as mentioned earlier, it is used to clear off the

vehicles in the intersection. Clearance interval is optional in a signal design. It depends on the geometry of the

intersection. If the intersection is small, then there is no need of clearance interval whereas for very large

intersections, it may be provided.

Cycle time

Cycle time is the time taken by a signal to complete one full cycle of iterations i.e. one complete rotation

through all signal indications. It is denoted by C. The

way in which the vehicles depart from an intersection

when the green signal is initiated will be discussed now.

Figure 50 illustrates a group of N vehicles at a signalized intersection, waiting for the green signal. As the

signal is initiated, the time interval between two vehicles, referred as headway, crossing the curb line is noted.

The first headway is the time interval between the initiation of the green signal and the instant vehicle

crossing the curb line. The second headway is the time interval between the first and the second vehicle

crossing the curb line. Successive headways are then plotted as in figure 5134:7. The first headway will be

relatively longer since it includes the reaction time of the driver and the time necessary to accelerate. The

second headway will be comparatively lower because the second driver can overlap his/her reaction time with

that of the first driver’s. After few vehicles, the headway will become constant. This constant headway which

characterizes all headways beginning with the fourth or fifth vehicle, is defined as the saturation headway, and

is denoted as h. This is the headway that can be achieved by a stable moving platoon of vehicles passing

through a green indication. If every vehicles require h seconds of green time, and if the signal were always

green, then s vehicles/per hour would pass the intersection. Therefore,

Where

S = saturated flow rate per hour of green time per lane

h = saturated headway in seconds

As noted earlier, the headway will be more than h particularly for the first few vehicles. The difference

between the actual headway and h for the ith

vehicle and is denoted as ei shown in figure 51. These differences

for the first few vehicles can be added to get start up lost time, l1 which is given by,

∑

The green time required to clear N vehicles can be found out as,

Figure 51Headways in Departing signal

Figure 50Group of vehicles at a signalized intersection waiting for

green signal

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Where

T = time required to clear N vehicles through signal

= start-up lost time

h = saturation headway in seconds

Effective green time

Effective green time is the actual time available for the vehicles to cross the intersection. It is the sum of

actual green time (Gi) plus the yellow minus the applicable lost times. This lost time is the sum of start-up lost

time (l1) and clearance lost time (l2) denoted as tL. Thus effective green time can be written as,

Lane capacity

The ratio of effective green time to the cycle length (

) is defined as green ratio. We know that saturation

flow rate is the number of vehicles that can be moved in one lane in one hour assuming the signal to be green

always. Then the capacity of a lane can be computed as,

Where

Ci is the capacity of lane in vehicle per hour,

Si is the saturation flow rate in vehicle per hour per lane,

C is the cycle time in seconds.

Critical lane

During any green signal phase, several lanes on one or more approaches are permitted to move. One of these

will have the most intense traffic. Thus it requires more time than any other lane moving at the same time. If

sufficient time is allocated for this lane, then all other lanes will also be well accommodated. There will be

one and only one critical lane in each signal phase. The volume of this critical lane is called critical lane

volume.

Determination of cycle length

Highway capacity manual (HCM) has given an equation for determining the cycle length C is given by,

∑( )

Where

N is the number of phases,

L is the lost time per phase,

V is the volume

C is the capacity.

(

) = ratio of volume to saturation flow for phase i,

= quality factor called critical V/C ratio

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Green splitting

Green splitting or apportioning of green time is the proportioning of effective green time in each of the signal

phase. The green splitting is given by,

[

∑

]

Where is the critical lane volume and is the total effective green time available in a cycle.

This will be cycle time minus the total lost time for all the phases. Therefore,

Where C is the cycle time in seconds, n is the number of phases, and is the lost time per phase. If lost time

is different for different phases, then cycle time can be computed as follows.

∑

Where is the lost time for phase i, n is the number of phases and C is the lost time in seconds. Actual green

time can be now found out as,

Where is the actual green time, is the effective green time available, is the amber time, and is the

lost time for phase i.

Pedestrian crossing requirements

Pedestrian crossing requirements can be taken care by two ways; by suitable phase design or by providing an

exclusive pedestrian phase. It is possible in some cases to allocate time for the pedestrians without providing

an exclusive phase for them. For example, consider an intersection in which the traffic moves from north to

south and also from east to west. If we are providing a phase which allows the traffic to flow only in north-

south direction, then the pedestrians can cross in east-west direction and vice-versa. However in some cases, it

may be necessary to provide an exclusive pedestrian phase. In such cases, the procedure involves computation

of time duration of allocation of pedestrian phase. Green time for pedestrian crossing Gp can be found out by,

Where, is the minimum safe time required for the pedestrians to cross, often referred to as the `̀ pedestrian

green time", = is the start-up lost time, is the crossing distance in meters, and is the walking speed of

pedestrians which is about 15th percentile speed. The start-up lost time can be assumed as 4.7 seconds and the

walking speed can be assumed to be 1.2 m/s

Performance measures

Performance measures are parameters used to evaluate the effectiveness of the design. There are many

parameters involved to evaluate the effectiveness of the design and most common of these include delay,

queuing, and stops. Delay is a measure that most directly relates the driver’s experience. It describes the

amount of time that is consumed while traversing the intersection. The figure 52 shows a plot of distance

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versus time for the progress of one vehicle. The desired path of the vehicle as well as the actual progress of

the vehicle is shown. There are three types of delay as shown in the figure. They are stopped delay, approach

delay and control delay.

Stopped time delay includes only the time at which the vehicle is actually stopped waiting at the red signal. It

starts when the vehicle reaches a full stop, and ends when the vehicle begins to accelerate.

Approach delay includes the stopped time as well as the time

lost due to acceleration and deceleration. It is measured as the

time differential between the actual path of the vehicle, and

path had there been green signal. Control delay is measured

as the difference between the time taken for crossing the

intersection and time taken to traverse the same section, had

been no intersection. For a signalized intersection, it is

measured at the stop-line as the vehicle enters the

intersection. Among various types of delays, stopped delay is

easy to derive and often used as a performance indicator and

will be discussed.

Vehicles are not uniformly coming to an intersection. i.e., they are

not approaching the intersection at constant time intervals. They

come in a random manner. This makes the modeling of signalized

intersection delay complex. Most simple of the delay models is

Webster’s delay model. It assumes that the vehicles are arriving at

a uniform rate. Plotting a graph with time along the x-axis and

cumulative vehicles along the y-axis we get a graph as shown in

figure 53. The delay per cycle is shown as the area of the hatched

portion in the figure.

Webster derived an expression for delay per cycle based on this,

which is as follows.

*

+

Where

= effective green time,

C= cycle length,

= critical flow for that phase,

S = saturation flow.

Delay Models in the HCM 2000

The delay model incorporated into the HCM 2000 includes the uniform delay model, a version of Akcelik’s

overflow delay model, and a term covering delay from an existing or residual queue at the beginning of the

analysis period. The delay is given as,

Figure 52Illustration of delay measures

Figure 53Graph between time and cumulative

number of vehicles at an intersection

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* +

* ( ) ( )+

[( ) √( )

]

Where,

d = control delay, s/veh

= uniform delay component, s/veh

PF = progression adjustment factor

= overflow delay component, s/veh

= delay due to pre-existing queue, s/veh

T = analysis period, h

X = v/c ratio

C = cycle length, s

k = incremental delay factor for actuated controller settings; 0.50 for all pre-timed controllers

I = upstream filtering/metering adjustment factor; 1.0 for all individual intersection analyses

c = capacity, veh/h

Level of service for signalized intersections

Level of service (LOS) for signalized intersections is defined in terms of control delay. The average control

delay is estimated for each lane group and aggregated for each approach and for the intersection as a whole.

Capacity and V/C ratio

The v/c ratio for each lane group is computed directly by dividing the adjusted flow by the capacities

computed as equation

Where

= Capacity of lane group I (vph)

= saturation flow rate for lane group i (vphg)

= green ratio for lane group i

C = cycle length (s)

Table 36 LOS criteria for signalized intersection

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The ratio of flow rate to capacity, v/c, often labeled as X is therefore:

(

)

( )

Where;

= v/c ratio for lane group i (vph)

= saturation flow rate for lane group i (vphg)

= actual flow rate for lane group i (vph)

= effective green time for lane group i (s)

C = cycle length (s)

A critical v/c ratio less than 1 indicates that all movements in the intersection can be accommodated within the

defined cycle length.

Example 9.1

The traffic flow for a four-legged intersection is as shown in figure 36:3.

Given that the lost time per phase is 2.4 seconds, saturation headway is 2.2

seconds, amber time is 3 seconds per phase, find the cycle length, green

time and performance measure(delay per cycle). Assume critical v/c ratio

as 0.9.

Example 9.2

Figure 54LOS of Signalized intersection methodology

Figure 55Traffic flow at four legged

intersection

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An intersection approach at an isolated pre-timed signal with a cycle length of 80 s has a saturation flow rate

of 3,000 veh/h. The length of the green is 24 s. The v/c ratio is 0.90. What is the level of service, if control

delay is measured over a 15 min interval?

9.3 Signal Co-ordination methods, Simultaneous, Alternate, Simple progressi on and Flexible

progression Systems.

Signal Co-ordination methods

For signals that are closely spaced, it is necessary to coordinate the green time so that vehicles may move

efficiently through the set of signals. In some cases, two signals are so closely spaced that they should be

considered to be one signal. In other cases, the signals are so far apart that they may be considered

independently.

Coordination attempts to achieve some combination of the following objectives:

Minimise fuel consumption

Minimise pollution emission

Minimise stops

Minimise delay

Maximise smooth flow

Maximise capacity

Minimise queue length

Minimise arrival of platoons at red lights

The signals less than 800m apart are coordinated as common practise. All but most complex coordination

scheme requires same cycle length for all signals.

Terminology

Offset: the difference between the green initiation times at two adjacent intersections. Usually expressed as a

positive number between zero and the cycle length. Sometimes convenient to think of it as a negative number,

usually no more than one half a cycle length.

Ideal offsets: The ideal offset is a value such that the first vehicle of a platoon just arrives at the downstream

signal, the downstream signal turns green.

( )

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Bandwidth: The amount of green time used by a continuously moving platoon of vehicles through a group of

intersections (time difference between the first and the last vehicle moving through the system without

stopping)

Traffic signal progression

Traffic signal progression is the concept of linking traffic signals together along a street so that "platoons" of

vehicles can pass through signalized intersections without getting stopped at a red light. Some factors that

affect traffic signal progression include signal spacing, speed of traffic, cycle length traffic signals run along

the corridor, and roadway congestion.

Forward and reverse progressions

Simple progression is the name given to the progression in which all the signals are set so that a vehicle

released from the first intersection will arrive at the downstream intersections just as the signals at those

intersections initiate green. As the simple progression results in a green wave that advances with the vehicles,

it is often called a forward progression (Figure 56). It may happen that the simple progression is revised two

or more times in a day, so as to conform to the direction of the major flow, or to the flow level. In this case,

the scheme may be referred to as a flexible progression. Under certain circumstances, the internal queues are

Figure 56 Coordinated Traffic Signal

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sufficiently large that the ideal offset is negative. The downstream signal must turn green before the upstream

signal, to allow sufficient time for the queue to start moving before the arrival of the platoon. The visual

image of such a pattern is of the green marching upstream, toward the drivers in the platoon. This is referred

to as reverse progression.

Simultaneous progressions

All signals show the same indication at the same time.

Provides inefficient timing

Increases speeds

Reduces capacity

Best suited to very short (300-500 ft.) or equally spaced very long blocks, and locations where Major Street

can have most of the green time.

Alternate System

Single Alternate. Every other signal shows the same indication.

Double Alternate. Every other pair of signals shows the same indication.

Requires 50-50% cycle split.

Not well-suited for unequal block spacing.

Double alternate reduces the through band width by 50%.

Best suited for downtown areas with square blocks and low speeds.

Figure 57Simultaneous Progression

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Progressive Systems

A signal system in which the vehicles receive a green indication as they arrive at the intersection.

Signal progression on one-way streets

Determining ideal offsets

Figure 58Single alternate progression

Figure 59Double alternate progression snlkh

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Assuming no vehicles are queued at the signals, the ideal offsets can be determined if the

platoon speed is known. For the purpose of illustration, a platoon speed of 60 km/h is

assumed. The offsets are determined according to

( )

Next the time-space diagram is constructed according to the following rules:

The vertical should be scaled so as to accommodate the dimensions of the arterial, and the

horizontal so as to accommodate at least three to four cycle lengths.

The beginning intersection should be scaled first, usually with main street green initiation at

t=0, followed by periods of green and red.

The main street green of the next downstream signal should be located next, relative to t=0

and at the proper distance from the first intersection. With this point located, the periods of

green, yellow and red for this signal are filled in.

This procedure is repeated for all other intersections working one at a time.

For the analysis let the signal phase cycle be as following:

Following table show the time needed to travel the link with uniform speed of 60 kmph.

Table 37Offset calculation

Link Distance

(m)

speed (60kmph)

in m/s

time (sec)

cumulative

Sallaghari - Thimi 2100 16.68 126 126

Thimi - Gathaghar 1300 16.68 78 204

Gathaghar - Kaushaltar 1400 16.68 84 288

Kaushaltar - Jadibuti 900 16.68 54 342

The above table shows that the vehicles passed through green interval of Sallaghari signal at t = 0 sec will

reach in Thimi after 126 sec that mean green interval should be in for their passing through intersection.

Similarly green signal should be on Gathaghar at t = 204 sec, on Kaushaltar at t = 288 sec and on Jadibuti at t

= 342 sec for the uniform.

One-way street progressive systems can provide a band width of 100% of green regardless of the block

spacing.

Figure 60Signal phase cycle

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Effect of vehicles queued at signals

It sometimes happens that there are vehicles stored in block waiting for a green light. These may be stragglers

from the last platoon, vehicles that turned into the block, or vehicles that came out of parking lots or spots.

The ideal offset must be adjusted to allow for these vehicles, so as to avoid unnecessary stops. The ideal offset

can then be given as:

( )

where, Q = number of vehicles queued per lane, veh, h= discharge headway of queued vehicle, sec/veh, and

Loss1 = loss time associated with vehicles starting from rest at the first downstream signal.

Signal Progression - on two-way streets and in networks

Consider that the arterial shown in Fig 61 is not a one-way but rather a two-way street. Fig. 62 shows the

trajectory of opposite vehicle on this arterial.

Figure 61Signal progression on one-way streets

Figure 62Vehicles movement from both direction

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Offset determination on a two-way street

If any offset were changed in Fig. 61 to accommodate the opposite vehicle(s), then the northbound vehicle or

platoon would suffer. The fact that offsets are interrelated presents one of the most fundamental problems of

signal optimization.

For longer lengths as in Fig. 63 the offsets might add to two cycle lengths. When queue clearances are taken

into account, the offsets might add to zero lengths. The general expression for the two offsets in a link on a

two-way street can be written as

Figure 63Offsets on 2 way arterials are not independent- Two cycle length

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CHAPTER TEN: TRAFFIC AND ENVIRONMENT

10.1 Detr imental effects of Traffic on Environment

Fuel efficiency or Fuel Economy is the energy efficiency of a vehicle, expressed as the ratio of distance

traveled per unit of fuel consumed in km/liter. Fuel efficiency depends on many parameters of a vehicle,

including its engine parameters, aerodynamic drag, weight, and rolling resistance. Higher the value of fuel

efficiency, the more economical a vehicle is (i.e., the more distance it can travel with a certain volume of fuel).

Fuel efficiency also affects the emissions from the vehicles. Fuel Consumption is the reciprocal of Fuel

Efficiency. Hence, it may be defined as the amount of fuel used per unit distance, expressed in liters/100km.

Lower is the value of fuel consumption, more economical is the vehicle. That is less amount of fuel will be

used to travel a certain distance.

Automobile Pollution

The pollution caused due to the emissions from vehicles is generally referred to as automobile pollution. The

transportation sector is the major contributor of air pollution. Vehicular emissions are of particular concerns,

since these are ground level sources and hence have the maximum impact on the general population. The

rapid increase in urban population have resulted in unplanned urban development, increase in consumption

patterns and higher demands for transport and energy sources, which all lead to automobile pollution. The

automobile pollution will be higher in congested urban areas. The vehicle obtains its power by burning the

fuel. The automobile pollution is majorly caused due to this combustion, which form the exhaust emissions, as

well as, due to the evaporation of the fuel itself. The chemical reactions occurring during ideal combustion

stages may be represented as follows:

( ) ( )

Similarly, the typical engine combustion which occurs in vehicles can be represented by the below chemical

equation.

( ) ( )

Types of Vehicular Emissions

The fuel loss of vehicles may be due to emissions or refueling. The emissions maybe evaporative or exhaust

emissions. The fuel losses in a vehicle are shown in Fig. 64.

Exhaust emissions: Exhaust emissions are those which are emitted through the exhaust pipe when the vehicle

is running or is started. Hence, the exhaust emissions maybe of 2 types - start up emissions and running

emissions.

Startup emissions: Emissions when the vehicle is started initially. Based on how long the

vehicle had been turned off after use, they may be cold start and hot start. Cold start refers to

when the vehicle is started suddenly after a long gap of use, whereas, hot start refers to when

the vehicle is started without the vehicle getting enough time to cool off after its previous use.

Running emissions: Emissions during normal running of the vehicle, i.e., when the vehicle is

in a hot stabilized mode.

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Evaporative emissions: These include running losses and hot soak emissions produced from fuel evaporation

when an engine is still hot at the end of a trip, and diurnal emissions (daily temperature variations).

Exhaust Pollutants

The pollutants which are emitted from the exhaust pipe of the automobiles are known as exhaust pollutants.

They are formed as a result of combustion of the fuel in the engine. These pollutants are harmful to the

atmosphere and living things in particular. The major types of exhaust pollutants are discussed in the

following sections.

The major air pollutant is total suspended particles (TSP) and PM10, due to the following main sources (in

approximate order of importance).

For TSP: resuspension from roads, bricks kilns. Domestic fuel combustion. Diesel vehicles, gasoline vehicles.

For PM 10: Diesel vehicles, gasoline vehicles, resuspension, domestic fuel, brick kilns.

Sulphur Oxides (SOx)

Combustion of petroleum generates Sulfur Dioxide. It is a colorless, pungent and non-flammable gas. It

causes respiratory illness, but occurs only in very low concentrations in exhaust gases. Further oxidation of

SOx forms H2SO4 and thus acid rains.

Nitrogen Oxides (NOx)

Combustion under high temperature and pressure emits Nitrogen dioxide. It is reddish brown gas. Nitrogen

oxides contribute to the formation of ground level Ozone and acid rain.

Hydrocarbons and Volatile Organic Compounds (HC and VOC)

Hydrocarbons result from the incomplete combustion of fuels. Their subsequent reaction with the sunlight

causes smog and ground level Ozone. V OCs are a special group of Hydrocarbons. They are divided into 2

types methane and non-methane. Prolonged exposure to some of these compounds (like Benzene, Toluene and

Xylene) may also result in Leukemia.

Carbon Dioxide (CO2)

It is an indicator of complete combustion of the fuel. Although it does not directly affect our health, it is a

greenhouse gas which causes global warming.

Carbon Monoxide (CO)

Figure 64 Losses of fuel in vehicles

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It is a product of the incomplete burning of fuel and is formed when Carbon is partially oxidized. CO is an

odorless, colorless gas, but is toxic in nature. It reaches the blood stream to form Carboxyhemoglobin, which

reduces the flow of Oxygen in blood.

Lead (Pb)

It is a malleable heavy metal. Lead present in the fuel helps in preventing engine knock. Lead causes harm to

the nervous and reproductive systems. It is a neurotoxin which accumulates in the soft tissues and bones.

Particulate Matter (PM)

These are tiny solid or liquid particles suspended in gas (soot or smoke). Particulate Matter in higher

concentrations may lead to heart diseases and lung cancer.

Factors Affecting Emission Rates

The vehicular emissions are due to a variety of factors. The emissions vary according to the environment, fuel

quality, vehicle, etc. emissions are higher in congested and urban areas. Fuel adulteration and overloading also

cause higher amount of emissions. The emissions from vehicles depend on the following factors:

Travel related factors

Highway Network related factors

Vehicle related factors

Travel Related Factors: The number of trips, distance travelled and driving mode are the major travel related

factors affecting emissions. As the number of trips increases, the amounts of emissions also increase.

Emissions increase with the distance travelled by the vehicle. The vehicular emissions also depend on the

driving mode. The driving modes may be idling, cruising, acceleration and deceleration. These modes

complete one driving cycle. Other factors affecting the emission rates are the speed, acceleration and engine

load of the vehicle. Low speeds, congested driving conditions, sharp acceleration, deceleration, etc. result in

higher emissions. On the other hand, intermediate speeds and low density traffic conditions cause lower

emissions.

Highway Network Related Factors: These include the geometric design features of the highway such as grade.

The emission rate is very high at steep gradients, as the vehicle needs to put in more effort to maintain its

speed. The highway network facilities such as signalized intersections, freeway ramps, toll booths, weaving

sections, etc. also influence the vehicular emission rates.

Vehicle related factors include the engine sizes, horsepower and weight of the vehicle. Vehicles with large

engine sizes emit more pollutants. Since larger sized engines are seen in vehicles with more horsepower and

more weight, these factors also contribute to the emission rates. Another important factor is the age of the

vehicle. Older vehicles have higher emission rates.

Other Factors

Ambient Temperature: Evaporative emissions are higher at high temperatures.

Type of engine: Two stroke petrol engines emit more amounts of pollutants than the four stroke diesel

engines.

Urbanization: Congestion is higher in urban areas, and hence emissions are also higher.

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10.2 Air pol lution; Noise Pol lution

Table 39WHO Guideline Value on Noise Level

Table 38WHO Guideline Value on Air Quality

Table 40Emission of TSP and PM10

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Table 41Noise Level at Different Areas

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Table 42Monthly Average of PM10 for 2003-2007 in Different Areas

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10.3 Measures to curtai l environme ntal degradation due to traffic

Following are the measures that will help to curtail environmental degradation due to traffic:

Create the balanced traffic: There should be balance between vehicular traffic and non-vehicular

traffic and pedestrian and road user.

Vehicle emission test and sound level test of vehicle: The vehicle should have been emission test as

well as sound level test. Those vehicle that exceed the critical level of emission and sound should be

banded.

Vehicle registration should be scientific: Vehicle registration with in the municipal should be

scientific based on the area, number of population and road network.

Traffic awareness regarding the degradation on environment due to traffic.

Very old as well as non-conditioned vehicles should be obsoleted. High fuel efficient vehicles

should be used.

The residue from garage such as used diesel etc should be well disposed.

Mass transit system: Sajha Yatayat vehicles replace about 4-5 micros buses and 10 tyampoo.

Greenery development along the roadside.

Sound absorber unit should be installed among the major city areas.

Development of non-pollution vehicle like trolley bus, electric bus and solar vehicles. The

then trolley bus service had been obsoleted from Nepal but one electromechanical cable car

has been developed for Manakamana temple visit.

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REFERENCES

Roess, RP., McShane, WR. and Prassas, ES. (1998), Traffic Engineering, Prentice Hall.

May, A. D. (1990), Fundamentals of Traffic Flow, Prentice Hall.

Papacostas, C. S. (1987), Fundamentals of Transportation Engineering, Prentice Hall.

Kadiyali, LR (1987), Traffic Engineering and Transportation Planning, Khanna.

Highway Capacity Manual (2000), Transportation Research Board, USA.

Khanna, S. K. and Justo, C. E. G. (1991), Highway Engineering, Nemchand.

Pingnataro, G. J. (1970), Principles of Traffic Engineering, Mc Graw - Hill.

npTEL note on Traffic Engineering and Management, Dr. Tom V. Mathew, IIT Bombay

Environment Statistic of Nepal, 2008, Government of Nepal, National Planning Commission

Secretariat, Central Bureau of Statistics, Kathmandu, Nepal

Signal Progression, Permit Writers Workshop

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