1

TRAFFIC STUDIES

2

Carried out to analyse traffic characteristics. Help in deciding geometric design features and traffic

control. Traffic surveys for collecting data are also called Traffic Census.

Various traffic studies are: Traffic volume study Speed study

Spot speed studySpeed and delay study

Origin and Destination (O&D) study Traffic flow characteristics Traffic capacity study Parking study Accident study

TRAFFIC STUDIES

3

TRAFFIC VOLUME STUDY

4

Traffic Volume :- Number of vehicles crossing a

section of road per unit time at any selected period.

One of the fundamental measures of traffic on a

road system.

Used as a quantity measure of flow.

Unit: vehicles per day and vehicles per hour.

TRAFFIC VOLUME

5

Measure of relative importance of roads for deciding the

priority for improvement & expansion.

Used in planning , traffic operation and control of existing

facilities and to design new facilities.

Analysis of traffic patterns & trends

Classified volume -structural design of pavements &

geometric design

For planning one way streets and other regulatory measures.

Turning movement study for design of intersection , signal

timings etc

Pedestrian traffic volume for planning of side walks,

pedestrian signals etc.

USES OF TRAFFIC VOLUME

6

Methods for volume counts are

1. Manual method

2. Combination of manual and mechanical method

3. Automatic devices

4. Moving Observer Method

5. Photographic method

TRAFFIC VOLUME COUNTS

7

Employs a field team to count and classify traffic

volume.

Procedure

No. of observers required depends upon no. of

Lanes and type of information required.

More desirable to record traffic in each direction

of travel by posting separate observers for each

direction.

For all-day counts ,work in three shifts of 8 hrs

can be arranged.

Data is recorded by Five- dash system. (IIII)

MANUAL METHOD

8

Details such as vehicle classification and no. of

occupants can be obtained.

Records Turning movement data of vehicles at

intersections.

Analyzing traffic characteristics in unusual conditions

such as adverse weather conditions, traffic

breakdowns, temporary closure of any lane of highway

etc.

Comparatively cheaper method of counting.

It is often necessary to check the accuracy of

automatic devices

Data accumulated by manual methods are easy to

analyze.

MANUAL METHOD- ADVANTAGES

9

Not practicable to have manual count for 24 hrs of

the day and throughout the year.

Counts of remote areas is possible by automatic

devices.

MANUAL METHOD- DISADVANTAGES

10

Multiple Pen Recorder

A chart moves continuously at the speed of a

clock. Different pens record different events.

Actuation of pen is by pressing the corresponding

electric switch.

When a particular class of vehicle passes ,the

associated switch may be pressed and this

operates the pen on the channel. This represents

the arrival of that particular class of vehicle.

COMBINATON OF MANUAL & MECHANICAL METHOD

11

MULTIPLE PEN RECORDER

ADVANTAGES

Permanent record is kept about arrival of each

class of vehicle.

Classification and vehicle count are performed

simultaneously.

12

1. Pneumatic Tube

Flexible tube with one end sealed is clamped to

road at right angles to pavement.

Other end connected to a diaphragm actuated

switch.

When an axle of vehicle crosses the tube, a volume

of air gets displaced thus creating a pressure which

closes the electrical contact through switch.

Two such contact result in one count.

AUTOMATIC DEVICES

13

PNEUMATIC TUBE

Disadvantages When vehicles with more than two axles

are present. Tubes get damaged by crawler tractors,

tyre chains etc. Difficulty in fixing them to gravel surfaces.

14

2. Electric Contact Devices

A pair of steel strips are contained in a rubber

pad which is buried beneath the surface.

When vehicle passes over rubber pad ,steel

strips come into contact with each other and

electric current flows.

Electric contact devices are installed in each

lane.

AUTOMATIC DEVICES

15

3. Coaxial Cables.

A coaxial cable is clamped across the road

surface,

signals are generated when vehicle passes over

it.

Signals actuate the counter.

Advantage : lesser susceptibility to damage.

AUTOMATIC DEVICES

16

4. Photo-electric Device. On one end of road , a source of light emits a

beam across the road. At the other end is a photo –cell which can

distinguish between the light beam and absence of light beam.

Passage of vehicle in the path of light beam obstructs the beam and detected by photo-cell.

Disadvantages Obstruction can be caused by pedestrians also. More than one vehicle positioned in the beam line

simultaneously can register as one vehicle.

AUTOMATIC DEVICES

17

5. Radar

Based on Doppler Effect.

Compares the frequency of a transmitted

radio signal with the frequency of the

reflected signal.

When a moving vehicle interrupts the

signal , a frequency difference exists.

AUTOMATIC DEVICES

18

Traffic Flow Maps: Show volume along various routes by using bands

proportional to traffic volume carried. Thickness of lines represent traffic volume.

Intersection Flow Diagrams Give direction and volume of traffic through an

intersection.

Trend charts Shows the hourly , daily or monthly changes in

volume through an area. Useful for planning future expansion, design and

regulation.

PRESENTATION OF DATA

19

Annual Average Daily Traffic(AADT)The total yearly volume divided by number of days in

the year. Average Daily Traffic(ADT)

The total volume during a given time period, in whole days greater than one day and less than one year, divided by number of days in that time period.

Thirtieth highest hourly volumeHourly 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.

High facilities are designed for 30th highest hourly traffic volume. Generally taken as the hourly volume for design.

Cost will be less.There will be congestion only during 29 times in a

year.

BASIC TERMS

20

SPEED STUDIES

21

Spot SpeedInstantaneous speed of a vehicle at a specified

location.

Average SpeedAverage of the spot speeds of all vehicles

passing a given point on the highway.

Running SpeedAverage speed maintained by a vehicle over a

given course while the vehicle is in motion.Running speed= Length of course / Running

time

= Length of course /( Journey time- Delays)

SPEED

22

Journey speed Also known as overall travel speed It is the effective speed between two points. It is the

distance between two points divided by the total time taken by the vehicle to complete the journey ,including all delays.

Journey speed = Distance/ Total journey Time (including Delays).

Time Mean Speed Average of instantaneous speeds of observed vehicles

at a spot over a period of time.

Space Mean speed Average of the speed measurements at an instant of

time over a space.

SPEED

23

Design of geometric features.

For regulation and control of traffic operations.

To establish safe speed limits.

Location and size of traffic signs.

For analyzing causes of accidents.

To study the traffic capacity, problems of

congestion on roads etc.

To decide speed trends.

USES OF SPOT SPEED

24

The methods available for measuring spot speed can be grouped as

Those observations that require time taken by a vehicle to cover a known distance. It consist of long base methods- vehicles are timed over a long

distance. Short base methods- vehicles are timed over a long

distance, say about 2m. Radar Speedometer which automatically records

instantaneous speed. Photographic method.

SPOT SPEED STUDIES

25

I). Those observations that require time taken by a vehicle to cover a known distance.

Long base methods Direct timing Procedure Enoscope Pressure contact tubes.

a)Direct timing ProcedureSimplest methodTwo reference points are marked on the pavement at a suitable distance apart and an observer starts and stops stopwatch as a vehicle crosses these two marks.

DIRECT TIMING PROCEDURE

26

From known distance and time, speed can be

calculated.

Disadvantage: errors due to parallax effect.

Observation by two observers one at each

reference points

First observer standing at the point in which the vehicle

pass first, signals and seconds observer starts the stop

watch. The second observer stops the stop watch when

he observes the same vehicle passing the reference

point.

Disadvantage: reaction time of two observers.

SPOT SPEED STUDIES

27

b). Enoscope

Also known as mirror box

L-shaped box, open at both ends ,with a

mirror set at 45 ˚ to the arms of the

instrument.

ENOSCOPE

28

45˚

observer

vehicle

mirror

ENOSCOPE

It bends the line of sight of the observer

perpendicular to the path of vehicle.

29

ENOSCOPE

30

The instrument is placed directly opposite to the first

reference point and the observer stations himself at the

second reference point.

Stop watch is started as soon as the vehicle passes the

first point and is stopped as soon as it passes the observer.

If two enoscope is used, the observer stations himself

midway between two reference points. Advantages:

Eliminates parallax error. Equipment is of low cost, simple and can set up easily

ENOSCOPE

31

Pneumatic tubes are used. At both reference points, a flexible rubber tube is

fastened to the pavement in perpendicular direction to the path of vehicle travel.

A vehicle crossing the tube compresses the air in it ,actuating an air impulse switch.

When the air impulse is sent, it activates an electromagnetically controlled stopwatch in the hands of observer.

When vehicle passes over second tube, stopwatch automatically stops.

Reading is noted by observer.

PRESSURE CONTACT TUBES

32

Advantage

Low initial cost

Simple to install

Easily maintained

Disadvantage

Tubes are easily seen by drivers and may affect

their behavior.

PRESSURE CONTACT TUBES

33

Works on the Doppler principle. Speed of a moving body is proportional to change in

frequency between the Radio wave transmitted to the moving body and the radio wave received back.

Instrument directly measures speed. Instrument is setup near the edge of carriageway at

a height of 1m.

The speed meter is so kept that the angle between direction of travel of vehicle and axis of transmission of radio wave is as low as possible, say within 20˚.

Speed of vehicles in both directions can be observed.

RADAR SPEED METERS

34

Time-lapse camera photography can be used.

Photographs are taken at fixed intervals 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 be also used.

PHOTOGRAPHIC METHOD

35

Average speed of vehicles

From spot speed data, distribution tables are

prepared by arranging the data in groups covering

various speed ranges and number of vehicles in

each range.

Arithmetic mean is the average speed.

Cumulative speed of vehicles

Graph is plotted with average values of each group

on X-axis and cumulative % of vehicles travelling

at different speeds.

PRESENTATION OF SPOT SPEED DATA

36

85th percentile speed: Speed at or below 85 % of

vehicles are passing the point of highway.

Only 15 % of vehicles exceed the speed at that

spot.

Drivers exceeding 85th percentile speed are

considered to drive faster than the safe speed.

Hence 85th percentile speed is adopted for Safe

speed limit at that zone.

50th percentile speed: median speed.

98th percentile speed: used for geometric design.

15th percentile speed: represents the lower

speed limit.

PRESENTATION OF SPOT SPEED DATA (contd..)

37

Modal average

A frequency distribution curve of spot speeds is

plotted with speed of vehicles or average values of

each group on the Y-axis. This graph is called

Speed Distribution Curve.

The peak value of the curve is denoted as Modal

Speed.

Modal speed: speed at which the greatest

proportion of vehicles move.

PRESENTATION OF SPOT SPEED DATA (contd..)