3A Traffic Studies

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KAAF UNIVERSITY COLLEGE

Civil Engineering DepartmentCollege of Engineering

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Transportation Engineering ICIV 367

Lecture 3A_ Traffic Studies

Kwasi Agyeman Boakye ( [email protected])

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Introduction

Traffic Studies is important to;

Managing the physical system, Investigating trends over time, Understanding the

needs and choices of the public and industry, Calibrating basic relationships or

parameters, Assessing the effectiveness of improvements, Assessing potential

impacts, Evaluating facility or system performance

It can be categorized under;

Inventories; Providing a list or graphical display of existing information, such as

street widths, parking spaces, transit routes, traffic regulation etc

Dynamic Studies; Involve the collection of data under operational conditions and

include studies of speed, traffic volume, travel time and delay, parking and crashes.Administrative Studies; Involve studies using existing engineering records,

available in government agencies and departments.

Examples;Volume studies, Speed studies, Travel Time studies, Delay studies,

Density Studies, Accident Studies, Parking Studies, Goods Movement and TransitStudies, Pedestrian Studies etc. 2


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Speed StudiesPurpose

Speed studies may be used for;

Establish parameters for traffic operation and control, such as speed zones, speed limits and

passing restrictions.

Evaluate the effectiveness of traffic control devices, such as a variable message signs at work

zones.

Monitor the effect of speed enforcement programmes such as the use of radar guns.

Evaluate and determine the adequacy of highway geometric characteristics such as radii of

horizontal curves and lengths of vertical curves.

Evaluate the effect of speed on highway safety through the analysis of crash data for different

speed characteristics.

Determine speed trends

Determine whether complaints about speeding are valid 4


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Manual Method of Measuring

Spot Speed

The main manual method used is the Direct timing procedure .

Direct Timing Procedure

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

marks. From the known distance and the measured time intervals speeds are

calculated.

Skilled observers can read a stop watch to an accuracy of 0.2sec. If the observer

stations himself inconspicuously, the speed readings are not influenced by driverreaction.

The disadvantage of this method is errors due to the parallax effect. Parallax is a

displacement or difference in the apparent position of an object viewed along two

different lines of sight. 5


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Manual Method of Measuring

Spot Speed

A simple device called Enoscope eliminates the parallax effect that creeps in when

the direct readings are taken by one observer.

The instrument bends the line of sight of the observer so that it is perpendicular to the

path of the vehicle.

The stop watch is started as soon as the vehicle passes the first reference point and

is stopped as soon as it passes the observer.

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Automatic Method of Measuring

Spot Speed

Automatic methods can be categorized under

1. Road detectors

2. Radar guns

3. Photographic and video camera

Road Detectors

They can be categorized into pneumatic road tubes and inductive loops. They can

also be used to collect volume data apart from speeds.

The advantage of the detector meters is that human errors are considerably reduced.

The following advantages are associated with them;

1. The devices are expensive

2. Where pneumatic tubes are used, they are rather conspicuous and may,

therefore affect driver behaviour, resulting in a distortion of the speed distribution.

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Automatic Method of Measuring

Spot Speed

Radar Guns

They work on the principle that when a signal is transmitted onto a moving vehicle,

the change in frequency between the transmitted signal and the reflected signal is

proportional to the speed of the moving vehicle.

The advantage of this method is that it can be located at an inconspicuous position

where the influence on driver behaviour is considerably reduced.

Photographic and video camera

Time lapse camera photography has been used successfully to determine the

speeds of vehicles accurately. By this method photographs are taken at fixedintervals of time (say one second per frame) on a special camera. By projecting the

film on a screen, the passage of any vehicle can be traced with reference to time.

Images by video cameras can also be used.

The major advantage is that one needs not to physically install a device like

pneumatic tube or loop on the road.8


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Analysis and PresentationSpot Speed

Average Speed : This is the arithmetic mean of all observed vehicle speeds ( which

is the sum of all spot speeds divided by the number of recorded speeds. It is given

by ;

Where = arithmetic mean

fi = number of observations in each speed group

ui= mid value for the ith speed group

Median: This is the speed at the middle value in a series of spot speeds that are

arranged in ascending order. 50% of the speed values will be greater than the

median; 50% will be lesser than the median

Modal Speed: This is the speed value that occurs most frequently in a sample of

spot speeds.

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Analysis and PresentationSpot Speed

The ith percentile spot speed : This is the spot speed value below which i percent

of the vehicles travel; for example, 85th percentile spot speed is the speed below

which 85 percent of the vehicles travel and above which 15 percent of the vehicles

travel.

Pace: This is the range of speed, usually taken at 10 mi/h that has the greatest

number of observations.Standard Deviation : It is a measure of the spread of the individual speeds.

S = standard deviation

= arithmetic mean

Ui = mid value of speed class i

fi = frequency of speed class i

N = Number of observations

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ExampleSpot Speed

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ExampleSpot Speed

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Volume Count Units I

The common types of counts include;

Average Annual Daily Traffic (AADT)

It is the average of 24 hour counts collected everyday in the year. It represents 1/365

of the total annual flow. It is often used for estimating the total travel on a road

system and for determining the gross annual revenues from road users. It is also

used as standard for pavement design and maintenance.

Average Daily Traffic (ADT)

It is the average of 24 hour counts collected over a number of days greater than 1 but

less than a year. They are often needed for measurement of current demand and

evaluation of existing traffic flow.

Peak Hour Volume (PHV)It is the maximum number of vehicles that pass a point on a highway during a period

of 60 consecutive minutes. PHVs are often used for

Functional classification of highways

Capacity analysis (lane number, signal time, channelization)

Development of parking regulations13


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Volume Count Units II

Vehicle Classification (VC)/ Classified Counts

Volumes are recorded with respect to the type of vehicles eg. Passenger cars, two-axle trucks,

or three-axle trucks. GHA has the following categories of vehicles Categories of Vehicles

Counted.jpg and a typical count sheet is Classified Count Sheet.jpg. They are important for

geometric design characteristics eg. Turning radii, maximum grades, lane widths etc. Also for

structural design of pavements. Example of PHV and Classified Count.docx

Passenger Car Units (PCU)/ Passenger Car Equivalence (PCE)

It converts the volume into equivalent passenger car units where traffic is composed of a

number of type of vehicles by using equivalency factors. It shows the impact that a mode of

transport has on traffic variables (such as headway, speed, density) compared to a single car.

They are often used in the determination of highway capacity.

American Unite Kingdom

private car (including taxis or

pick-up) 1

motorcycle 0.5

bicycle 0.2

horse drawn vehicle 4bus, tractor, truck 3.5

Motorbike 0.4

Car 1.0

LGV 1.0

RGV 1.9

Articulated goods vehicle 2.9PSV (bus) 2.5

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Volume Count Units III

Vehicle Miles of Travel (VMT)

This is a measure of travel along a section of road. It is the product of the traffic

volume and the length of road way to which the volume is applicable. VMTs are used

mainly as a base for allocating resources for maintenance and improvement of

highways. It is also used in determining level of vehicle emissions.

Find out the following;

What is passenger km and when it is often used.

What is Passengers per bus hour and when it is often used.

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Methods of Conducting Volume

Counts I

There are two main methods of conducting volume counts which are namely; Manual

Counts and Automatic Counts. Benefits of each include

Manual Counts

Details such as vehicle classification and number of occupants can be easily

obtained. Unusual conditions obtained during time of count can be recorded.

In developing countries where automatic devices are hard to come by this method is

comparatively cheap.

They easily compliment and can serve as a check on the accuracy of data collected

by automatic means.

Automatic Counts

Where continuous and regular record of traffic flow is needed, the only alternative isto use automatic counts

If counts of remote areas are needed, automatic devices are useful

They yield data which is amenable to easy analysis by computers.

What are the disadvantages of both methods?

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Manual Counts I

Involves one or more persons recording observed vehicles using a counter.

Considerations

Link counts should be located on straight sections of road for good visibility. Duration

can be from a few minutes to several days, depending on purpose. Most counts are

carried out for one day, starting before the morning peak hour, and extending for 12,14 or 16 hours. Count periods are usually 15 minutes, with results summarised

hourly.

In Turning movement counts the vehicle classification system is simplified (to

compensate for the extra demands on Surveyors). These surveys are primarily

concerned with the performance of the junction during peak periods. Count periods

are usually 15 minutes. However, at signalised junctions short-term volumes are

determined by signal operations, and cycle times in excess of 2 minutes, or count

periods less than 15minutes, may cause apparent fluctuations in flow. In such cases

it is best to record flow for each cycle, using the cycle time as the counting period.

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Manual Counts II

Survey Forms

Pencil and Paper method; -The 'five-bar gate' method is the most versatile and is

applicable to both link and turning counts. It records data quickly(without the need for

extreme neatness), is the most economical in use of space on the survey form and

can be summarised quickly.

Alternatively, hand tallies can be used to record specific vehicle classes. They arefaster and more accurate than pencil and paper methods, because only the total is

recorded, at the end of each count period, and the surveyor is looking at the traffic

flow almost continuously. However, vehicles can be missed while recording the

cumulative total at the end of each count period.

Equipment Needed

1. A watch 2. Pencils, eraser and pencil sharpener 3. Supply of blank field data

sheets 4. Clipboards

Errors in Manual Counts

1. Failure to define vehicle classification unambiguously

2. Failure to observe time periods accurately

3. Surveyors having to count vehicles at a faster rate. 18


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Manual Counts III

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Automatic Counts I

Automatic traffic counter equipment consists of a detector, to detect the vehicle, and

a counter to record the information. The most common types of detector used in

traffic counting are a hollow rubber pneumatic tube, held to the road surface by

special clips, or an inductance loop (several turns of wire laid on or in the road

surface in a rectangular or diamond shape).

Pneumatic Tubes

Pneumatic tubes detect the movement of a vehicle as the tube is depressed by a

tyre; this creates a pulse of air which closes a diaphragm in the recorder. The count

site must not be located where braking or acceleration occurs, or where overtaking is

common, such as near junctions, bus stops, or other parking locations.

Why should pneumatic tubes not be located where braking or acceleration occurs or

where overtaking is common?

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Automatic Counts IIPneumatic Tube

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Automatic Counts IIIInductive Loops

A loop detector detects the presence, rather than the movement of a vehicle.

The counter passes an alternating electric current through the loop, creating a

magnetic field above the loop. The presence of a metal vehicle in the magnetic field

causes a change in the electrical inductance, which is recorded by a sensor in thecounter.

Non-metallic vehicles cannot be detected and pedal cycles are very difficult to detect

because of their shape. Loops are usually permanent installations, laid in a shallow

slot cut into the road surface, and sealed in by epoxy resin and bitumen. However,temporary loops can be fixed with sticky tape to the road surface.

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Automatic Counts IVInductive Loops

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Types of Volume Counts

Cordon Counts

A cordon is an imaginary boundary around a study area. It is generally established to

define a CBD or other major activity centre where the accumulation of vehicles within

the area is of great importance in traffic planning. The intersection of each street

crossing the cordon line is taken as a count station; volume counts of vehicles or

persons entering and leaving the cordon area are counted.

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Cordon counts are useful for

Planning parking facilities

Updating and evaluating traffic operational techniques

Making long range plans for freeway and arterial facilities

Screen Line Counts

In screen line counts the study area is divided into large sections by running

imaginary lines, known as screen lines, across it.

Natural and man made barriers such as rivers or railway tracks are often used.

Traffic counts are taken at each point where a road crosses the screen line. It is

important for the screen line not to be traversed by the same road several times.

Often these counts are done to determine the variations in the traffic volume and

traffic flow direction due to change in land use pattern of the area. 25


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Intersection Counts

They are taken to determine vehicle classification through movements and turning movements

at intersections. The data obtained from this count is useful for ;

Determining phase lengths and cycle time for signalised intersections.

Design of channelisation at intersections

Design improvements of intersectionsPedestrian Volume Counts

Volume counts of pedestrians are made at locations such as intersections, along sidewalks and

midblock crossings, overpasses, underpasses etc.

Data from this count could be used for crash analysis , capacity analysis, and determining

minimum signal timings at signalised intersections.

Periodic Volume CountsTo make reasonable estimates of annual traffic volume characteristics on an area wide basis,

different types of periodic counts, with count durations ranging from 15 minutes to 60 continuous

,are conducted. The various periodic counts conducted include continuous, control and

coverage counts.

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Example of Network Count

A represents the continuous count station

1-6 represents the control stations 1 6.

Two set of crew to do the counting from 12pm to 8pm.

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Scenario 1 One Day Study

Assumptions-One crew will be used to count control location A for the entire 8 hour period of the study.

-The second crew would be used to count each of the coverage locations 1 6 for 1 hour

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Scenario 1 One Day Study

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Scenario 2 Six Day Study Count

Assumptions

-Each coverage station is to be counted for 8 hours.

-One crew monitors the control station and the other crew 8 hours each day on each coverage

station.

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Scenario 2 Six Day Study Count

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Periodic Counts

Continuous Counts. These counts are taken continuously using mechanical or electroniccounters. In selecting stations where continuous counts are done (permanent count stations)

the highways within the study area must first be properly classified. Thus links should be

categorized according to similar traffic characteristics such as AADT and daily, weekly and

seasonal variations in traffic volumes.

Control Counts. These counts are taken at stations known as control count stations whichare strategically located so that representative samples of traffic volume can be taken on each

type of highway or street in an area wide traffic counting program. The data obtained from

control counts are used to determine seasonal and monthly variations of traffic characteristics

so that expansion factors can be determined. Control counts can be divided into major and

minor control counts. Major control counts are taken monthly, with 24-hr directional counts taken

on at least three days during the week ( Tues, Wed, Thurs) and also on Saturday and Sunday

to represent the weekend.Coverage Counts. These counts are used to estimate ADT, using expansion factorsdeveloped from control counts. The study area is usually divided into zones that have similar

traffic characteristics. At least one coverage count station is located in each zone. A 24-hr non

directional weekday count is taken at least once every 4 years at each coverage station. It tells

changes in area wide traffic characteristics.

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Adjustment of Periodic Counts

Expansion factors, used to adjust period counts, are determined either from continuous count

stations or from control count stations. Hourly, daily, and monthly expansion factors can be

determined using data obtained at continuous count stations.

Hourly Expansion Factors (HEF)

These factors are used to expand counts of duration shorter than 24-hr to 24-hr volumes by

multiplying the hourly volumes for each hour during the count period by the HEF for that hour

and finding the mean of these products.

HEF = total volume for 24-hr period

volume for particular period

Daily Expansion Factors (DEF)These factors are used to determine weekly volumes from counts of 24-hr duration by

multiplying the 24-hr volumes by the DEF

DEF = average total volume for the week

average volume for particular day

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Monthly Expansion Factors(HEF)

The AADT for a given year may be obtained from the ADT for a given month by multiplying this

volume by the MEF

MEF= ________AADT____________

ADT for particular month

Example of Daily Expansion Factors

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Monthly Expansion Factors (MEF)

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Presentation of Volume Data

Traffic Flow Maps

These maps show traffic volumes on individual routes. The volume of traffic on each

route is represented by the width of a band, which is drawn in proportion to the traffic

volume it represents, providing a graphic representation of the different volumes that

facilitates easy visualisation of the relative volumes of traffic on the different routes.

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Intersection Summary Sheet

These sheets are graphic representations of the volume and directions of all traffic

movements through the intersection

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Time Base Distribution Charts

These charts show the hourly, daily, monthly or annual variations in traffic volume in

an area or on a particular highway.

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Documents

3A Traffic Studies