History and Perspectives of 85th Percentile Speed Limits

Research Lead: Ribeka Toda

UC Campus(es): UCLA

Problem Statement: 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.

Project Description: This study explores the research and evidence to support using the 85th percentile rule for setting the speed limit, particularly in urban areas. The study reviews the evolution of the rule and its application in cities primarily using interviews conducted with engineers, law enforcement officers, local political leadership, and legislators in the City of Los Angeles. This research is important because it informs the way we set speed limits in the state of California and in many other states across the country. The topic of speed limits is of current interest to the California legislature. Speed is a key factor in roadway safety and speed limits have a great influence on the roadway speed. In order for cities like Los Angeles to achieve their Vision Zero goals, speed must be analyzed critically to inform how we set and enforce speed limits.

Status: Completed

Budget: $10,176