8. Traffic Analysis

8.1 The Traffic System

The road and the vehicle are subject to engineering design and thus the characteristics of these components can be dictated to a large extent by the engineer. However the traffic engineer is essentially concerned with the road system and therefore the vehicle component is substantially beyond the scope of control of the traffic engineer.

The characteristics of the road user are obviously beyond the control of the traffic engineer, and these characteristics must therefore be accepted and catered for by the traffic engineer. To enable traffic design and management to be undertaken effectively, the traffic engineer requires a knowledge of human performance characteristics and vehicle characteristics. The road user may be involved with the traffic system as a driver, passenger or pedestrian but it is usually as a driver that is of most concern in traffic engineering.

8.2 The Driving Task

• navigation;
• guidance; and
• control.

Of course not all drivers are identical in their capabilities or habits. Driver behaviour seems to vary between individuals according to two factors: ability and motivation. Behaviour is dependent upon both what the driver is able to do and what the driver chooses to do. As a consequence, there is little correlation between driver skill and driver crash experience.

Driver ability is closely linked to prior experience. An experienced driver knows what effects any controlling action is going to have and is thus able to select appropriate actions, as well as to exercise greater discrimination in information input and processing. Experience allows for the development over time of a set of workable expectancies, which allow for anticipation and forward planning. If these expectancies are violated problems are likely to occur, either as a result of wrong decision or of an inordinately long reaction time.

Driver expectancy can be considered in three categories:

• Continuation expectancy – that the events of the immediate past will continue e.g. in a stream of traffic moving at reasonable speed it is not expected that the vehicle ahead will suddenly change speed or stop.
• Event expectancy – that events which have not been observed to happen will not happen e.g. if a driver has regularly crossed over a railway level crossing and never encountered a train no train is expected, and the level of risk may increase.
• Temporal expectancy – that where events are cyclic (e.g. traffic signals), the longer a given state occurs, the greater the likelihood that change will occur. This may result, perhaps, in drivers accelerating towards green traffic signals in the expectancy that they must turn red soon.

• driver’s expectations are recognised, and unexpected design or operational situations are avoided or minimised;
• predictable behaviour is encouraged through familiarity and habit; and
• information provided decreases the driver’s uncertainty.

8.3 Reaction Time

Reaction time may be considered to be comprised of four elements:

• perception – the use of sensory organs to detect the stimulus;
• identification – the identification and understanding of the stimulus;
• emotion – the driver deciding what action should be taken in response to the stimulus (e.g. apply the brakes, turn the steering wheel, etc.); and
• volition – executing the action decided upon.

Traffic system design and operation should aim to present to drivers situations that are simple and expected so that reaction times may be kept at low values. Some ways in which this may be done are:

• by encouraging familiarity – drivers will react slower to unfamiliar situations (e.g. unusual intersection layouts or non-standard traffic signs);
• by minimising the number of alternatives from which the driver must choose – a large number of possible actions for the driver is likely to lead to confusion and uncertainty e.g. multiway intersections with three or four possible routes to select from are more confusing for the driver than a Tee intersection;
• by providing positive information – the driver should be told what to do rather than what not to do e.g. ‘Wrong Way Go Back’ is a more positive message than‘Do Not Enter’; and
• by providing prior warning – the driver is prompted to expect an event which will require an action e.g. roadworks warning sign. The prior warning should be in the context of the action required e.g. a roadworks warning sign should be located where the roadworks are visible.

8.4 Visual Characteristics of Drivers

8.4.1 Visual Field

If a visual signal is to be seen it must obviously be within the driver’s visual field. For reading purposes the visual field is quite narrow, usually between 3° and 10°. However, objects outside this field can be detected by peripheral vision which extends to about 90° left and right, 60° above and 70° below the line of sight. These values are for a stationary observer and tests show that the values are reduced when the observer is moving.

8.4.2 Eye and Head Movement

The main constraint on the rate of information gathering is the rate at which the eye can move from one object to another, and refocus. For normal driving, where the driver is performing several tasks simultaneously, a rate of 1.0 to 1.5 fixations per second would be reasonable. Thus, for traffic design, it is necessary for ‘signals’ to be separated in time. As the driver is usually sampling inputs from a moving vehicle this also means that the signals must be separated in space. For example a driver travelling at 100 kph, and sampling at the rate of 1.0 to 1.5 fixations per second, would need to have the inputs spaced at about 20 to 28 m apart. If the ‘signals’ (signs, linemarkings, traffic signals, etc) are closer than this, some information will be missed because the driver is physically incapable of sampling at a faster rate.

8.4.3 Illumination

The human visual system is capable of operating over an enormous range of illumination. Of interest in traffic engineering is the eye’s ability to adjust to fairly rapid changes in light intensity. On exposure to glare after a dark situation, the pupil diameter contracts at a rate of about 3 mm/s, whereas on exposure to dark after glare it is much less responsive, dilating at about 0.5 mm/s. In other words the eye can adjust to sudden glare more rapidly than sudden dark. This is important when designing artificial lighting for tunnels, where greater time must be allowed for the driver’s eyes to adjust as the tunnel is entered than when the tunnel is exited.

8.4.4 Visual Handicaps

Several visual handicaps may have an effect on driver behaviour. About 2.5% of the adult male population has colour defective vision, such that they cannot discriminate red, yellow and green (as in traffic signals), or indeed any three-colour combination. Another 2.5% of the adult male population has a reduced sensitivity to red light. Approximately 5% of the population are visually deficient with respect to detecting low luminance contrasts. Visual sensitivity decreases with age and the detection threshold of elderly drivers is about double that of ‘normal’ drivers. It is interesting to note that no correlation has been found between poor visual performance and driver safety, suggesting that drivers with visual impairment compensate in their driving behaviour.

8.5 The Information Needs of Road Users

• conspicuity i.e. the ‘signal’ must be seen;
• legibility i.e. the message must be able to be read;
• comprehensibility i.e. the message must be understood; and
• credibility i.e. the message must be perceived to be true.

8.6 Factors Modifying Normal Driver Behaviour

Fatigue

Fatigue is a decrease in the body’s work output or psychological or emotional feelings. The body adopts a state between that of being wide-awake and being asleep, and is best described as a state of drowsiness. Fatigue may result from monotony, from an adverse environment (e.g. from a closed, warm atmosphere), from over-work, from emotional factors (e.g. worry) and from physiological factors (e.g. over-eating). The symptoms of fatigue are loss of attention to a task and boredom. From a driving viewpoint the results of fatigue may be decreased visual scanning, increased response times and falling asleep while driving. Fatigue due to emotional or physical causes can only be overcome by rest and recuperation. If the cause of fatigue is organic, such as narcolepsy, relief will only be achieved by medical treatment.

Alcohol

Alcohol acts as a depressant on the central nervous system of the body. When alcohol is orally taken into the body as a fluid it travels to the small intestine where the main absorption into the blood stream occurs. The alcohol is then spread to all parts of the body, including the brain where it has major effects. In small amounts alcohol may act as a relaxant and can give the sensation of improved mood, but judgement and decision making processes deteriorate. With large amounts of alcohol muscle co-ordination and reflexes become slower, vision and hearing are impaired, and the brain’s ability to process information is diminished. Once alcohol has been absorbed into the blood stream it is metabolised by the liver into waste products. The process of removal of alcohol from the body is relatively slow and alcohol in the body is likely to affect driver performance for several hours. All States and Territories in Australia have laws which limit the amount of alcohol in the bloodstream (the blood alcohol concentration, BAC) for drivers.

Drugs

It has long been known that alcohol affects driving skill but it is only in fairly recent times that researchers have concentrated their efforts on looking at the effects of other drugs on driving performance. It is known that about half of the top 30 medications prescribed by doctors can affect driving, as well as many medications that can be purchased without a doctor’s prescription. As well as these drugs used for legitimate medical purposes, there are other drugs which are used by certain people for mood altering effects or for the symptoms produced by the development of physical dependence. These drugs include cannabis, cocaine, heroin and morphine, as well as hallucinogenic substances such as L.S.D.

8.7 Road Vehicles

The types of motor vehicles likely to be encountered on roads are passenger cars and their derivatives (e.g. station wagons), utilities and light vans, heavy vehicles such as trucks and buses, road trains and motor cycles.

The manoeuvrability of a vehicle is closely related to its overall size, length, width, height and mass. It is accepted practice that roads be designed and constructed to accommodate vehicles up to the legal maximum size, except in special circumstances.

LINKS TO SITES ON ROAD VEHICLES.

8.8 The Nature of Traffic Flow

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:

• Macroscopic Approach. This considers the flow in an aggregate sense. It is appropriate for studying steady state phenomena of flow and is best in modelling the overall system. The approach makes use of physical analogies to traffic flow such as heat flow and fluid flow.
• Microscopic Approach This approach considers the response of each individual vehicle in a disaggregate manner. The behaviour of individual driver vehicle combinations is modelled, and the approach has been used extensively in examinations of road safety.
• Human Factors Approach This approach seeks to define the mechanism by which individual drivers and their vehicles located themselves with reference to the rest of the road traffic system. This approach is closely related to the microscopic approach.

8.9 Parameters Describing 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:

• Volume (or flow rate) q. The number of vehicles passing a fixed point in unit time. Typical units are veh/day, veh/hr or veh/sec.
• Speed (or velocity) v. The distance travelled by a vehicle in unit time. Typical units are km/h (also kph) or m/s.
• Density (or concentration) k. The number of vehicles per unit length of lane or road, at a given time instant. Typical unit is veh/km.
• Headway. The time gap between successive vehicles in a traffic stream (actually between the same points on the vehicles, e.g. front of vehicle). Typical unit is sec. "
• Spacing. The distance between the same physical point (e.g. front of vehicle) on two successive vehicles in a traffic lane. Typical unit is m.
• Occupancy. The proportion of time that a designated point in a traffic lane is covered by vehicles.

8.10 Speed-Volume Relationship of Traffic

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).

8.11 Types of Traffic Facilities

Traffic facilities may be classed into two broad categories:

• Uninterrupted Flow Facilities. On these facilities vehicles operate without interruption by external factors. The traffic flow therefore depends only on the interaction between individual vehicles and between vehicles and the road. Typical uninterrupted flow facilities would be two lane rural roads and freeways.
• Interrupted Flow Facilities. On these facilities vehicle operation is likely to be controlled by factors external to the driver and the roadway. For example stop signs and traffic signals cause traffic to stop and hence interrupt the flow. Typical interrupted flow facilities would be major urban roads and urban streets.

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.

8.12 Capacity

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:

• capacity is usually expressed as persons/hour or vehicles/hour;
• the time period used to determine capacity may be less than one hour e.g. 15 minutes;
• if capacity is being analysed for a section of roadway (the usual case), then prevailing roadway, traffic and control conditions should be reasonably uniform for the section being analysed;
• roadway conditions refer to the geometric characteristics of the road e.g. type of road, number of lanes, lane width, shoulder width, design speed, horizontal alignment and vertical alignment;
• traffic conditions refer to the characteristics of the traffic stream using the road e.g. vehicle types, lane distribution, directional distribution; and
• control conditions refer to the types and specific design of the control devices, and traffic regulations.

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.

8.13 Level of Service

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:

• Level of Service A. A condition of free flow in which individual drivers have the freedom to select their desired speed. Drivers are virtually unaffected by the presence of other vehicles in the traffic stream. The general level of comfort and convenience provided is excellent.
• Level of Service B. Drivers still have reasonable freedom to select their desired speed and to manoeuvre within the traffic stream. However the general level of comfort and convenience is a little less than with level of service A.
• Level of Service C. The flow is still quite stable but most drivers are restricted to some extent in their freedom to select their desired speed and to manoeuvre within the traffic stream. The general level of comfort and convenience declines noticeably at this level.
• Level of Service D. This is close to the limit of stable flow and is approaching unstable flow. All drivers are severely restricted in their freedom to select their desired speed and to manoeuvre within the traffic stream. The general level of comfort and convenience is poor.
• Level of Service E. This occurs when traffic volumes are at, or close to, capacity. There is virtually no freedom for drivers to select their desired speed, or to manoeuvre within the traffic stream. Flow is unstable and a minor disturbance within the traffic stream may cause the flow to break down.
• Level of Service F. This is the zone of forced flow. The amount of approaching traffic is greater than that which can pass and so queuing and delays occur.

The concept of level of service may be used to analyse the operation of all types of road facilities.

8.14 Factors Affecting Capacity and Level of Service

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:

• Roadway Conditions;
• Terrain Conditions;
• Traffic Conditions; and
• Driver Population.

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:

• uninterrupted single lane flow;
• uninterrupted two lane two way roads;
• uninterrupted multi lane roads;
• freeways;
• urban arterial roads with interrupted flow;
• unsignalised intersections; and
• signalised intersections.

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.

8.15 Uninterrupted Two-Lane, Two-Way Roads

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:

• analysis of general terrain segments – the segments usually being 3 km or longer and having reasonably uniform roadway, terrain and traffic conditions;
• analysis of specific grades – generally with grades greater than 3 percent and longer than 1 km; and
• analysis for planning purposes.

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:

• design speed is 100 km/h or greater;
• traffic lanes are 3.7 m wide or greater;
• clear shoulder widths are 2.0 m or greater;
• sight distance along the road is always greater than 450 m;
• traffic consists of passenger cars only;
• a 50/50 directional split of traffic occurs;
• no restrictions occur due to traffic control or turning vehicles; and
• terrain is level.

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 28 June 2010.