Fast, Furious & Fatal: An Assessment of Speed Setting Methodology in California (brief)

Abstract

As vehicle speed increases, so do the probability and severity of a crash. Recognizing this relationship, the City of Los Angeles updated its speed limits in 2017 as part of its Vision Zero program. Yet California’s methodology for setting speed limits, known as the 85th percentile rule, forced LA to increase speed limits on more than 90 miles of streets with a history of known collisions.A common practice in the United States, the rule sets the speed limit at the speed at which 85 percent of the vehicles travel at or under on a given roadway. Supporters of the 85th percentile methodology see it as a safe and fair way to set the speed limit based on the driving behavior of the majority of drivers. Critics of the methodology disagree with setting speed limits based on existing driver behavior, claiming that it will create unsafe road conditions. I examined California’s methodology, its relationship to roadway safety (especially in urban areas), and other possible approaches that could improve safety.