C. CAMERON
Research Officer, Australian Road Research Board
THE DETERMINATION OF PRIORITIES AT
CONTROLLED AND UNCONTROLLED
INTERSECTIONS-RIGHT OF WAY
AIM OF THIS PAPER
1.
In recent months there have been
several statements suggesting that what is
popularly known as the 'give way to the
right' law is an unsatisfactory method of
resolving conflicts between vehicles at intersections. These statements are not, as yet,
supported by sufficient data on traffic flow
and accidents to justify a conclusion that the
present law is either unsafe or productive of
delays.
2.
The purpose of this short paper is
to examine the concept of priorities implied
in the law, in the light of what is already
known about the operation of the law and
about the behaviour of drivers in the type
of situation which the law is intended to
resolve.
POPULATION EXPECTANCY
3.
The existence of a 'population
expectancy' consistent with a major-minor
system of priorities has been postulated by
several authorities (Ref. 1, 3, 5 and 8). It
implies a tendency for traffic entering a high
volume road from a lower volume 'side
street' to behave as though the high volume
had some degree of priority. Field studies
employing delay measures have, in general,
failed to show the existence of such a tendency (Ref. 4 and 7), but the possibility
that its existence would not be reflected in
this type of measure remains (Ref. 7). At
least one investigation in which a single
intersection was intensively studied found
evidence of a secondary major-minor expectancy, which exerted some influence on
the behaviour of drivers, although a giveway-to-the-right tendency appeared to be
dominant (Ref. 1). The evidence so far
available does not permit a clear conclusion
on this point, especially as the number of
intersections at which there is a large dif38
ference between intersecting volumes, and
at which there is no form of traffic control,
is rather small. It is only under such conditions that the postulated major-minor behaviour would be expected to occur. The
existence of two conflicting expectancies,
one reinforced by the existing law and the
other by everyday driving experience, may
tend to weaken both, and thus tend to increase the variability and unpredictability of
behaviour at intersections.
GIVE WAY — TO THE RIGHT
OR TO THE LEFT?
4.
The choice of a give way to the right
instead of a give way to the left (which has
often been said to be more appropriate when
traffic moves on the left of the carriageway)
has resulted, at least in theory, in a reinforcement of major-minor expectancies.
With the requirement to give way to the first
traffic stream which he meets, a driver is
more likely to enter an intersection slowly,
or to stop before doing so, and continuity
of flow on the higher volume road is protected. Although this aspect of the system
is not well known, the intention of preserving continuity of high volume flow in this
way is quite deliberate, and is viewed by
some traffic engineers as a means of obtaining some of the advantages of a majorminor system without accepting its disadvantages.
THE DECISION PROCESS
5.
No attempt has so far been made
to analyse the decision processes of the
driver in determining whether to give or
accept right of way. It is a matter of common observation that considerable uncertainty exists, especially when three or more
vehicles are in conflict. Car A is obliged to
give way to B on his right; car B in turn is
AUSTRALIAN ROAD RESEARCH
CAMERON - DETERMINATION OF PRIORITIES AT CONTROLLED AND
UNCONTROLLED INTERSECTIONS - RIGHT OF WAY
required to give way to car C on his right
and therefore cannot accept his right of way
over car A; but car C intends to turn right
across A, and therefore has to give way to
A. This has been called, following the terminology used in statistical ranking methods,
an intransitive situation. A not uncommon
result is that car D, on the fourth arm of the
intersection, arrives after the others and
proceeds straight through, leaving A, B and
C no better off than before.
This is only one example of the
6.
complex integrated movement of several
vehicles which is often required under the
give way rule. If the perceptual judgements
involved in assessing the movements of
vehicles approaching the intersection are
also considered, it may be seen that the
decision process is a complex one. The more
complex the decision, the longer is the
period of hesitation, and the effect of this
on traffic delays can be estimated from
empirical observation at intersections of different volumes.
It is sometimes suggested that a cer7.
tain amount of hesitation is a factor contributing to safety. It may tend to ensure that
an intersection is approached at a slow speed
and the driver's heightened anxiety level
may be regarded as irrelevant. This is
another matter which can only be resolved
empirically; this has not yet been done.
MAJOR-MINOR ROADS AND PRIORITY ROADS
8.
Major-minor roads are sometimes
confused with a priority road system, and it
may be of value to outline the difference
between them. Continuity of flow on a
priority road is protected at intersections
over a considerable distance and the road
may be regarded as limited-access'. In a
major-minor system a decision about which
of two roads is the major one is made independently at each intersection, so that Road
A may be major at the point where it intersects with Road B, and minor at its intersection with Road C. A major road is
merely a road which has priority at a given
intersection.
Volume 3, No. 6, June, 1968
SAFETY ASPECTS
There is evidence that modification
9.
of the give way to the right law in Victoria
in 1963 was followed by a significant rise in
collisions at intersections (Ref. 6). It would
be unsafe to conclude that the law is therefore a more dangerous one to operate. The
modified law altered the obligation to give
way to vehicles already in the intersection,
and it is again a matter of common observation that many drivers were not fully conversant with this change for some time after
its introduction. In any case, a rise in collisions at intersections cannot be regarded
as evidence in favour of a major-minor system, since no figures are available for such
a system. In terms of safety, the case for or
against the give way rule, as compared with
a major-minor system, must be regarded as
not proven.
COMPATIBILITY OF THE TWO SYSTEMS
10.
In the discussion so far, and indeed
in the public statements mentioned already,
there has been an implicit assumption that
the give way to the right law and a majorminor system are mutually exclusive. There
appears to be no real reason why the two
methods of control cannot co-exist within
the same traffic system, as primary and
secondary methods of resolving right of way
priorities. There is already some evidence
that they operate in this way in the actual
behaviour of drivers (Ref. 1).
11.
For the two systems to co-exist, it is
necessary to accept a distinction between
controlled and uncontrolled intersections.
At controlled intersections, movements are
controlled by traffic signals, give way signs,
stop signs or flashing lights. Although only
the give way sign alters the legal obligation
of the driver on the left to give way to avoid
a collision, in practice they all modify driver
behaviour in such a way that they can be
regarded as devices to determine priorities.
The give way rule does little to resolve conflicts at these intersections, and they might
with advantage be designated major-minor
situations,. with give way signs on the minor
road. (Except for the traffic signal case,
39
CAMERON - DETERMINATION OF PRIORITIES AT CONTROLLED AND
UNCONTROLLED INTERSECTIONS - RIGHT OF WAY
where traffic is under close control at all
times.) At uncontrolled intersections priorities might be determined by the give way
rule; in general these would be low volume
intersections at which there is no great difference between volumes on intersecting
streets. It is already accepted among traffic
engineers that the give way rule does not
adequately control movements at high
volume intersections, although the limiting
conditions beyond which a more refined
system of control is required do not seem to
have been defined.
12.
At present, a driver who has a 'Give
Way' sign in his favour at an intersection is
not provided with knowledge of this fact.
He has to infer it from previous experience
of the intersection or from a glimpse of the
back of the sign on the intersecting road. If
a mixed system of the type suggested in this
paper were to be introduced, some means
of providing this knowledge would have to
be devised, if only to avoid a dangerous
confusion between an uncontrolled intersection, at which the give way law determines
priorities, and an intersection with a minor
road. This presents no great problem;
methods in use overseas might well be appropriate in Australia.
A reasonable aim for research into
13.
right of way problems appears to be the
determination of boundary conditions within
which a give way law gives adequate control, and outside which more sophisticated
control is necessary. This appears to require
mathematical modelling of a range of intersecting traffic flows to determine the levels
at which the probability of a (legally) unresolvable situation becomes unacceptable.
This probability level would be determined
by the overall effect on traffic flow through
the intersection.
The information from such a study
14.
might be used to determine warrants for
the designation of an intersection as uncontrolled or controlled. The task of the motorist would be no harder than it is at present,
since he is already required to recognize and
observe whatever traffic control devices exist
at an intersection. In fact, his task might
40
be easier, because the required types of
traffic control would be reduced. The possibilities would be (a) traffic signals, (b) give
way signs, and (c) none.
RIGHT OF WAY AND GAP ACCEPTANCE
15.
The suggestion put forward for the
integration of major-minor and give way
to the right systems of control is consistent
with the gap acceptance ideas developed by
Blunden (Ref. 2) and others. By treating
busy intersections as major-minor situations,
the risk of encouraging a driver to exercise
his legal 'rights' in the absence of a suitable
gap is avoided. It is unrealistic to expect a
driver to try to enter a traffic stream if no
suitable gap exists, even if he has the full
majesty of the law on his side. Similarly,
it is unrealistic to expect drivers in the
`main stream' to create gaps into which
intersecting traffic may proceed. This is particularly so when the 'minor road' driver is
attempting to cross, rather than join, a
multi-lane stream of traffic. A driver in the
right hand lane who stops to permit entry
from the 'minor road' is likely to obstruct
the entering driver's view of the other traffic
lanes. Since the entering driver is understandably reluctant to proceed unless he
can see that all lanes of traffic are giving
way, the result is frequently a period of
hesitation, after which the main stream
driver abandons his attempt to give way and
drives on. On occasion, the movements of
the conflicting vehicles become uncoordinated and a collision or near collision may
result.
CONCLUSION
16.
The evidence for or against the various views which have been examined in this
paper is so limited that it would be presumptuous to attempt to draw any firm conclusions. It does appear, however, that there
is some evidence that giving way to the right
as required under the existing law is not
completely consistent with the expectancies
of drivers, which tend to be affected to some
extent by an implicit major-minor concept.
The possibility of taking advantage of this
by integrating the give way rule and a
AUSTRALIAN ROAD RESEARCH
CAMERON - DETERMINATION OF PRIORITIES AT CONTROLLED AND
UNCONTROLLED INTERSECTIONS - RIGHT OF WAY
major-minor system of traffic control should
at least be examined. It might well lead to
a simpler system of road signing, as well as
to a simpler decision process for the individual driver. The first step appears to be
an examination of the effect of traffic
volume, and perhaps other traffic variables,
on the probability of occurrence of intransitive situations. An appropriate method of
study is computer simulation of traffic flow.
Such a study would, in any case, be of value
in determining more clearly the warrants for
the introduction of traffic control devices
such as signals and give way signs.
REFERENCES
1. Barlow, G. W. and May, K. A. Driver right-of-way behaviour, University of
Melbourne, Unpublished Report (1968).
2. Blunden, W. R., The driver and the traffic stream, Symp. People on the
Road — Human Factors in Road Safety, Proc. A.N.Z.A.A.S. (1967).
3. Braybrooke, E. K., Problems of law enforcement, Proc., Symposium on
Traffic Hazards and the Community, University of W.A. (Oct. 1967).
4. Bryant, J. F. M., Report by Secretary to 8th Meeting, Human Factors Committee, A.R.R.B. (Feb. 1968).
5. Cumming, R. W., Capabilities and limitations of a driver, Proc., Australian
Study Week on Road Safety Practices, 3 (Melbourne — May 29/June 2,
1967).
6. Harper, B. C. S., The effects of legislation, Proc., Australian Study Week on
Road Safety Practices, 1 (Melbourne — May 29/June 2, 1967).
7. McGill, W. A., Right of way — some preliminary investigations, Aust. Rd
Res. (in press).
8. Pretty, R. L., Minutes, 11th Meeting, Traff. Engng Committee, A.R.R.B.
(Dec. 1966).
Volume 3, No. 6, June, 1968
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