How do traffic engineers determine speed limits?

Legal speed limits are established by law (in Arizona) 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 percent of the vehicles are traveling on the highway. Experience has shown that approximately 85 percent of motorists drive at a speed that is reasonable and prudent. Established speed limits therefore encourage voluntary compliance because they appear reasonable to the public. Those 15 percent of drivers who will not comply with reasonable speed limits are the drivers who are subject to enforcement action.