Fast, Furious & Fatal: An Assessment of Speed-Setting Methodology in California

Project Description

The 85th percentile rule is a widespread rule of thumb used for setting speed limits on public roads. Traffic engineers conduct a speed survey on a roadway and set the speed limit at the speed at which 85 percent of the drivers are driving at or under. Developed in the 1960s for use on rural two-lane highways in the US, this rule has been increasingly criticized as an inappropriate method for setting speed limits, especially in urban areas. The reasons behind the acceptance and continued use of this rule vary and are not uniform between engineers, law enforcement officers, and legislators.