General Concepts
The maximum speed limits posted as the result of a study should
be based primarily on the 85th percentile speed, when adequate speed
samples can be secured. The 85th percentile speed is a value that
is used by many states and cities for establishing regulatory speed
zones.
Speed checks should be made as quickly as possible, but it
is not necessary to check the speed of every car. In many cases,
traffic will be much too heavy for the observer to check all cars.