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Speed limits
A.M. Arman (Department of Transport) in his paper The Value
of Realism in Setting Speed Limits makes the following
definition:
“A satisfactory speed limit is a legal, and hence enforceable,
constraint on the maximum speed at which a driver of a motor
vehicle is permitted to travel. It conveys no guarantee that the
prescribed limit is attainable by all or any vehicle, in all or any
circumstances. It is not a recommended speed and it does not
indicate a safe speed. It is simply a legal maximum speed. A speed
limit is a crude device inasmuch as it is attached to a road 24
hours of every day and takes no account of variations in traffic
conditions, weather, visibility or the vehicles to which it
applies. At the same time, however, it is a most delicate device
which if improperly tuned, will not only fail to perform its design
function but even contribute to the very problems it is intended to
resolve.”
Not many people would disagree with the statement that traffic
speeds should be reduced. If it was simply a question of imposing
local speed limits which drivers automatically observed, then there
is little doubt we would have local speed limits on most of our
roads.
Local speed limits are not about speeds we wish to see. They are
not even about speed limits, which are desirable for the route.
They are about restrictions, which will work: that is actually
reducing traffic speeds. Speed limits that do not do this are
worthless.
Something more than fear of prosecution is required of a speed
limit as the object of the exercise is not to criminalise the
activities of most motorists.
The criteria (DoT Circular – Roads 1/93) states:
“… specific speed limits cannot, on their own, be expected to
reduce vehicle speeds if they are set at a level substantially
below that at which drivers would choose to drive in the absence of
a limit”.
Local speed limits can only work, therefore, if they succeed in
influencing driver behaviour. It is important that speed limits
have credibility and much of the criteria is concerned with driver
psychology – will most drivers consider the speed limit reasonable
and observe it!
There are two main criteria that need to be considered are:
- 85 percentile speeds
- development.
Development
Drivers react to the character of the environment of a road. A
30mph speed limit is generally associated with built up areas with
development in depth on both sides of the road, properties with
direct access on the road and large number of pedestrians. The
character of a 40mph speed limit can be similar or can be partially
built up (usually exceeding 50% of frontages) with conditions
typical of the transition between built up and out of town
areas.
85 percentile speeds:
Contrary to popular belief, local speed limits should only be
used if 85 out of 100 vehicles are already travelling at the speed
it is wished to impose. Experience shows the speeds of these 85
vehicles are likely to influence the speeds of the other 15. This
makes the speed limit largely self-enforcing and consequently makes
for a manageable enforcement task.
To sum up – the message is that inappropriate local speed limits
are not only ineffective but also burden the Police with the task
of trying to enforce the unenforceable. Perhaps more importantly an
unrealistic speed limit can lead to an increase in differential
speeds – leading to more overtaking manoeuvres which in turn can
lead to more accidents.
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