12. Traffic Studies
12.1 Purposes of Traffic Studies
A traffic study is the collection and analysis of
measurable factual data relating to traffic and its characteristics.
Traffic studies are carried out to:
- provide a basis for planning and designing traffic
facilities, including the selection of geometric standards, economic analysis,
and the determination of priorities;
- assist traffic operation by determining the need
for traffic control devices such as signs, traffic control signals, pavement
markings, and school and pedestrian crossings;
- evaluate the effect of changes made for traffic
by conducting before and after studies; and
- determine the basic characteristics and the general
laws of traffic behaviour.
12.2 Traffic Volume Counts
Traffic volume counts are made to determine the number
of vehicles passing a point. These counts may be comprehensive counts covering
the entire main road system in an area, counts on all roads intersecting
a cordon line which encircles a particular area, counts on screen line(s)
which divide a city into two or more parts, or counts at specific points.
The information sought may include traffic volume and the direction of traffic,
volume of turning traffic at intersections, hourly, daily, and seasonal variations
of traffic, or proportion of cars, trucks and buses.
Traffic volume counts can be carried out either
manually or by automatic traffic counters. Manual counts are usually undertaken
at points where it is necessary to record the proportion of vehicles as well
as the traffic volume, at intersections where the volume of the various turning
movements is required, or at sites being investigated for installation of
pedestrian crossings and guard-controlled crossings.
Equipment used in automatic counts normally consists
of a device to detect the passage of axles or vehicles, and a metering device
to record the number of axles or vehicles detected. The detecting device most
commonly used in Australia is the pneumatic detector. A rubber tube is laid
across the roadway and as a vehicle’s wheels pass over it an air impulse
is sent along the tube to the meter. Two main types of counter are used to
measure traffic volumes, the non-recording counter and the recording counter.
The non-recording counter accumulates the number of vehicles detected and
must be read at regular intervals over the time it is desired to obtain traffic
counts, e.g. if 24-hour counts are required the counter must be read every
24 hours. The recording counter records the traffic volume passing each hour
(in some cases each quarter hour, or each five minutes) by printing numbers
on, or by punching holes in, a moving paper tape, by recording on magnetic
tape, or by recording in a microprocessor which is later downloaded to a
computer.
The method of presenting results will depend on
the purpose of the study. In comprehensive area traffic counts the Average
Annual Daily Traffic (AADT) volumes are usually presented as traffic flow
maps in which flow bands proportional to the AADT volumes are drawn along
the route surveyed.
12.3 Origin and Destination Surveys
Traffic counts give the amount of traffic passing
specified points on the road but they do not indicate where traffic desires
to travel, i.e. its origin and its destination. An origin and destination
survey is designed to obtain such information about the movement of vehicles
and passengers within an area. The survey is primarily for transportation
planning, particularly the location, design, and programming of new or improved
highways, public transport, and parking facilities. An origin and destination
survey may range from a relatively simple study to determine the amount of
traffic that would by-pass a town to a comprehensive transportation survey
for planning and design of the transportation system in a large metropolitan
area.
Methods used for Origin and Destination surveys
include:
- Recording registration numbers;
- Handing postcards to drivers;
- Roadside interviews;
- Tag-on-vehicle surveys;
- Headlight surveys; and
- Home interview surveys.
12.4 Speed Studies
Vehicle speeds can be measured in the following ways:
- using a radar meter, which gives a direct reading
of speed for each vehicle;
- taking photographs of a section of road at a
predetermined time interval t, measuring the distance x covered by each vehicle,
and computing speed from: v = x/t
- timing successive vehicles, or a representative
selection of vehicles, over a short predetermined distance of length a.
Results may be presented in tables, graphs and diagrams.
These may include speed distribution and cumulative frequency distribution
curves. The more important quantities are the mean speed and the 85th percentile
speed.
12.5 Travel Time and Delay Studies
A modified form of the speed study is the travel
time and delay study. It measures the average journey time and journey speed
of sections of a route (or routes) and is used in traffic assignment, to
assess the quality of the traffic route, or to evaluate the before and after
effect of traffic engineering techniques or other changes made for traffic.
By analysing the delays, the location and cause of the congestion can be
identified and remedied.
12.6 Crash Studies
The establishment of an accurate and efficient crash
records system is of prime importance to the traffic engineer. Crash
records are used by the traffic engineer:
- to identify hazardous locations and the causes
of crashes at them;
- as an aid in determining priorities for road
improvements or the installation of traffic control devices;
- to evaluate safety improvements by before and
after studies;
- as a measure of the level of service provided;
and
- to carry out fundamental research into the relationship
between traffic crashes and the road environment.
Sources of data on crashes are reports of individual
crashes, and the information recorded varies from State to State.
Crash data may be conveniently represented by:
- Crash Spot Maps
An crash spot map shows the location of each
crash and the type of crash by coloured pins, e.g. fatal crashes
by red pins, injury crashes by blue pins, etc. The maps which are large
scale and attached to display boards are kept up to date as crash report
forms are received. The maps provide a quick visual guide of where crash
concentrations exist.
- Collision Diagrams
A collision diagram illustrated pictorially, by
means of directional arrows and symbols, the path and nature of collision
of vehicles and pedestrians involved in crashes at a particular intersection
or stretch of road. The diagrams are not usually drawn to scale, and distances
between high frequency locations may be shortened. The arrows do not show
exact paths as they would overlap and become confusing. Accordingly, crashes
of the same type are grouped together. A study of the crash pattern portrayed
by a collision diagram may suggest possible improvements such as installation
of traffic signals, channelisation, removal of obstructions etc. which may
eliminate a certain type of collision.
- Crash Summary Forms
The use of a standard crash summary form is
recommended when routine crash histories are required. The form contains
the same type of information for each crash occurring at a requested
location between specific dates. Although not having the visual impact of
a collision diagram a crash summary form provides more detail.
12.7 Parking Studies
Parking studies are carried out to indicate:
- the number and location of existing parking spaces,
both kerbside and off-street;
- existing parking practices, including usage of
available spaces, parking duration, illegal parking;
- the need to impose or vary parking time limits
or to install parking meters; and
- the adequacy of existing enforcement measures.
A suitable method for carrying out a parking survey
in a small city is as follows:
- Select the study area. This should include
the portions of the town or city containing most of the business activity
and should also include those areas that generally cater for parking.
- Carry out a parking inventory in the study
area. This involves the collection or preparation of suitable base maps of
the study area showing all streets, lanes, etc., the location and type of
all buildings and vacant areas, all kerb space available for legal parking
(this is obtained from a detailed survey), areas where vehicles cannot legally
park (e.g. close to intersections, school or pedestrian crossings, etc.),
any off-street parking lots, garages, or service stations with details of
available spaces, fees, etc.
- Carry out a cordon count. All vehicles entering
and leaving the study area should be counted at 15 or 30 minute intervals
for the period from say 7.00 am to 7.00 pm. Determine the number of vehicles
accumulated in the area at any particular time and the time of day when
it occurs. In small towns cordon counts may be omitted.
- Carry out a parking usage study. This study
should extend from say 8.00 am to 6.00 pm, the active hours of the day. Observers
should tour each kerb and parking lot or garage every 15 or 30 minutes and
record the registration numbers and type of vehicles at each parking space.
Illegally parked vehicles should also be recorded. Close to certain generators
such as a post office, it may be necessary to make checks at shorter time
intervals (particularly if 30 minutes has been used as the general interval).
The area can be divided so that the survey can be spread over several days
which have a similar parking demand. At off-street parking stations the actual
distribution of the parking usage can be obtained from the tickets.
For larger cities, a comprehensive parking demand
study may be required, which includes the determination of parking usage,
parking habits as well as the origin, destination, and purpose of trip of
drivers parking in the area. It is used primarily in determining the demand
for parking space by evaluating the individual parker’s desires. The actual
survey is carried out in the form of questionnaire cards or direct interviews.
LINKS TO SITES ON TRAFFIC STUDIES.
The Australian Transport Safety Bureau has produced an
informative report titled "The History of
Road Fatalities in Australia" with an overview of data covering the period
1925 to 1998.
Page last modified 28 May 2009.