8. Traffic Analysis
8.1 The Traffic System
The traffic system can be considered to be made up
of three components, namely the road, the user and the vehicle. For the
system to operate without failure the three components must interact in a
compatible manner. In practice this does not always occur with the result
that the system breaks down. Road accidents, congestion and traffic intrusion
are examples of system breakdown and in most cases result from an incompatibility
between the three components, or between one of the components and the environment
within which the system operates.
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
Driving can be considered as comprising three essential
tasks:
- navigation;
- guidance; and
- control.
These tasks require the driver to receive inputs,
process them, make predictions about the results of alternative actions,
decide which is the most appropriate action, and execute the action. The
driver then observes the effect of the act, gathers new information, and
repeats the sequence. There are many problems inherent in this sequence of
tasks which arise from the capabilities of the human driver and the interactions
between the driver and other components of the road traffic system.
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.
If the driver receives information in the expected
form, and events occur in accordance with that information, then the driver’s
performance is likely to be error free. Alternately, when the information
received does not match the drivers expectations, system failures are likely
to occur. The traffic engineer should therefore attempt to ensure that:
- 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 refers to the period between the occurrence
of stimulus to the driver and the driver’s physical reaction to it.
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.
Expectations reduce reaction times because drivers
respond through familiarity and habit. However, different drivers will have
different reaction times to the same stimulus because reaction time is affected
by a wide range of individual characteristics, such as experience, skill,
motivation, etc. Studies of driver reaction time have shown that for many
situations an average reaction time is about 2.5 seconds, but variations from
this average are quite large.
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
As previously mentioned the driving task is information-driven
and this requires the driver to select and sample inputs from the road traffic
system. About 90 percent of the information used by average drivers is visual
and a small amount of information is received by auditory or tactile means.
Since vision is so important to the driving task it is necessary to understand
the visual characteristics and limitations for design purposes.
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
The successful operation of the traffic system depends
to a large extent on successfully conveying information to drivers to aid
them in the driving task. The key needs of road users in relation to information
are:
- 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
There are three major influences which may cause
significant changes to a drivers normal driving behaviour. These influences
are fatigue, alcohol and drugs.
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
Study of traffic behaviour requires a detailed knowledge
of the characteristics of the types of vehicles commonly found in the traffic
stream. In particular properties such as dimensions, visibility restrictions,
manoeuvrability, acceleration, braking, grade climbing, steering, cornering,
driver vision, lighting, axle loads and axle spacing are important for the
design of roadway elements.
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.
The
Australian Design Rules
set out design standards for vehicle safety and emissions in Australia.
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:
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.
Top
Page last modified 28 June 2010.