Research Team: PI: Ribeka Toda Team:

About this project:

This study will explore the research and evidence to support using the 85th percentile rule for setting the speed limit, particularly in urban areas. The study will review 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 can inform 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.

What problem does this research aim to address?

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.

What are the expected impacts and benefits of the research?

The City of Los Angeles has adopted a Vision Zero goal to eliminate all traffic deaths by 2025. The Los Angeles Department of Transportation has identified a network of streets, the High Injury Network (HIN), where strategic investments will have the biggest impact in reducing deaths and severe injuries and is working to implement those investments. Speed limits and speed enforcement also affect roadway injuries and fatalities, and this research project informs City of Los Angeles strategic decision-making about pursuing safety-related changes to state laws on how speed limits are set.

Final Report
By |October 3, 2019|Categories: Traffic