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Traffic flow is a complex phenomena. The three main components of the road traffic system are the road, the user and the vehicle. These components all interact with each other. Consequently the moving traffic stream has characteristics which are quite different to those of the individual elements.
Traffic flow is concerned with the movement of discrete units (such as vehicles or people) around a network. In general, these units move independently of each other, although they interact. Each unit is usually under the control of a human operator, and the processes by which a traffic stream works can often be described in terms of random behaviour. The randomness originates from the multitude of individual decisions that occur in a traffic stream, where each human operator has some personal freedom of choice and action.
Three main approaches are available to the quantification and modelling of traffic flow:
There are at least six basic variables or measures used in describing traffic flow, and several other stream characteristics are derived from these. The three primary variables are speed (v), volume(q) and density(k). Three other variables used in traffic flow analysis are headway (th), spacing (s) and occupancy .
These terms can be defined as follows:
Average speed and volume are the more common descriptors of a traffic stream as they can be easily measured.
The three basic parameters are related to each other by the continuity of flow equation:
q = kv
in which v is the space mean speed. This equation only applies to the case of uninterrupted traffic flow (e.g. major highways or freeways).
Traffic facilities may be classed into two broad categories:
Where interruption to a flow occurs because of traffic signals it will be found that vehicles tend to 'bunch' or 'platoon'. This bunching occurs when vehicles are facing a red signal. When the green signal appears these vehicles move off as a bunch which will gradually disperse if the flow is not interrupted again. It is generally recognised that if traffic signals are spaced 3 km or more apart, some uninterrupted flow will develop. It should be noted that uninterrupted flow and interrupted flow describe the type of road facility, and not the quality of traffic flow on the road.
Capacity is defined as the maximum hourly rate at which persons or vehicles can reasonably be expected to pass a point (or uniform section of a lane or roadway) during a given time period under the prevailing roadway, traffic and control conditions.
The following points should be noted with respect to this definition:
The capacity of a road is an important characteristic. Roads are generally not expected to operate at or near capacity for long periods, because operating conditions at capacity are poor. Thus the ability to analyse the traffic carrying ability of facilities under better operating conditions is a major aspect of capacity analysis.
A qualitative measure describing traffic operational conditions and their perception by drivers is needed to assess the degree of congestion on a road. Such a measure is referred to as a 'level of service' and is intended to take account of factors such as speed and travel time, freedom to manoeuvre, traffic interruptions, comfort and convenience and safety.
Six levels of service are used for describing traffic flow conditions. These are designated from A to F with level of service A representing the best operating condition and level of service F the worst.
The levels can be generally described as follows:
The concept of level of service may be used to analyse the operation of all types of road facilities.
For the analysis of capacity or level of service the starting point is often to select values that are applicable to ideal conditions and then to apply correction or adjustment factors that reflect the actual roadway, traffic and control conditions. In general, an ideal condition is one for which further improvements will not result in any increase in capacity or level of service.
The factors affecting capacity and level of service include the following:
The NAASRA (1988) publication Guide to Traffic Engineering Practice, Part 2 Roadway Capacity provides details for analysing the capacity and service volume of a variety of facilities including:
In this module only the case of the uninterrupted two lane two way road situation will be discussed, and will be based on the approach presented in the NAASRA (now Austroads), publication.
Two lane rural roads have one lane available for traffic travelling in each direction. Overtaking of slower vehicles requires use of the opposing traffic lane, when convenient.
At low traffic volumes, drivers are able to choose their desired speed and overtaking of slower vehicles is usually accomplished with minor, if any, delay. As volume increases the need to overtake to maintain desired speed also increases, but the opportunities for overtaking decrease due to an increased traffic flow of oncoming vehicles. It is found that vehicles then tend to cluster in platoons or bunches.
Three types of analysis can be considered:
For two lane, two way roads, ideal conditions occur when no restrictions due to roadway, terrain and traffic conditions apply. Specifically, ideal conditions occur when:
If all of these conditions are fulfilled the capacity of a two lane two way road is 2800 passenger cars per hour. This is the total of both directions of flow.
Page last modified 24 June 2002.