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A traffic study is the collection and analysis of measurable factual data relating to traffic and its characteristics.
Traffic studies are carried out to:
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.
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:
Vehicle speeds can be measured in the following ways:
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.
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.
The establishment of an accurate and efficient accident records system is of prime importance to the traffic engineer. Accident records are used by the traffic engineer:
Sources of data on accidents are reports of individual accidents, and the information recorded varies from State to State.
Accident data may be conveniently represented by:
An accident spot map shows the location of each accident and the type of accident by coloured pins, e.g. fatal accidents by red pins, injury accidents by blue pins, etc. The maps which are large scale and attached to display boards are kept up to date as accident report forms are received. The maps provide a quick visual guide of where accident concentrations exist.
A collision diagram illustrated pictorially, by means of directional arrows and symbols, the path and nature of collision of vehicles and pedestrians involved in accidents 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, accidents of the same type are grouped together. A study of the accident 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.
The use of a standard accident summary form is recommended when routine accident histories are required. The form contains the same type of information for each accident occurring at a requested location between specific dates. Although not having the visual impact of a collision diagram an accident summary form provides more detail.
Parking studies are carried out to indicate:
A suitable method for carrying out a parking survey in a small city is as follows:
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.
The Federal Office of Road Safety in Australia has produced an informative report titled Monograph 23 "The History of Road Fatalities in Australia" and this is available from their Monograph Series page.
Page last modified 7 July 1999.