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The speed studies are accompanied for eminently logical purposes that will
influence what traffic engineering measures are implemented in any given case.
The location at which speed measurements are taken must conform to the
intentional purpose of the study.
The guiding philosophy behind spot speed studies is that measurements should
include drivers freely selecting their speeds, unaffected by traffic
congestion.
For example if driver approaches to a toll plaza, then he has to slow his
speed, so this is not suitable location to conduct the study, measurements
should be taken at a point before drivers start to decelerate.
Similarly, if excessive speed around a curve is thought to be contributing to
off-the-road accidents, speed measurements should be taken in advance of the
curve, before deceleration begins.
It may also be appropriate, however, to measure speeds at the point where
accidents are occurring for evaluation with approach speeds.
This would allow the traffic engineer to assess whether the problem is
excessive approach speed or that drivers are not decelerating sufficiently
through the subject geometric element, or a combination of both.
A study of intersection approach speeds must also be taken at a point before
drivers begin to decelerate.
This may be a moving point, given that queues get shorter and longer at
different periods of the day.
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