Legal speed limits are established by law and may be changed only when justified on the basis of an engineering study.
A widely accepted principle is to set speed limits as near as practicable to the speed below which 85% of the vehicles are traveling on the highway. Experience has shown that approximately 85% of the motorists drive at a speed that is reasonable and prudent.
Speed limits thus established encourage voluntary compliance because they appear reasonable to the public. Those 15% of drivers who will not comply with reasonable speed limits are the drivers who are subject to enforcement action.
Impact of Posted Speed Limits On Actual Traffic Speeds
There is a common belief among laymen, and even by some officials, that the mere posting of speed limit signs will cause drivers to react accordingly. This is not true and is why posted speed limits must be realistic to receive compliance.
Unrealistically low speed limits will invite violation by responsible drivers. Enforcement of unreasonably low limits sets up the so-called "speed trap" that results in poor public relations. The posting of proper speed limits has the beneficial effects of smoothing traffic flow and aiding effective law enforcement.