10. Road Users and Vehicles
10.1 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.
10.2 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.
10.3 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.
10.4 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.
10.4.1 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.
10.4.2 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.
10.4.3 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.
10.4.4 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.
10.5 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.
10.6 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.
10.7 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.
Page last modified 3 July 2003.