What is the 85th-percentile rule?

Traffic engineers rely on the 85th-percentile rule to establish speed limits. The speed limit is set to the speed that separates the bottom 85 percent of vehicle speeds from the top 15 percent. Engineers theorize that traffic laws reflecting the behavior of the majority of motorists have better compliance than laws that arbitrarily criminalize the majority of motorists and encourage violations. The latter kinds of laws lack public support and often fail to bring about desirable changes in driving behavior. An example is the federally mandated 55 mph speed limit that was repealed in part because of notoriously low compliance.

Most U.S. jurisdictions report using the 85th-percentile speed as the basis for their speed limits, so the 85th-percentile speed and actual speed limits should be closely matched. However, a review of available speed studies shows that the posted speed limit is usually set below the 85th-percentile speed by as much as 12 mph. Political or bureaucratic resistance to higher limits and statutes that restrict jurisdictions from posting limits higher than an arbitrary number may be causes.

Traffic Ticket Information | Find a Traffic Ticket Attorney