Analyzing the strengths and weaknesses of the 85th percentile rule

Status

Complete

Project Timeline

July 1, 2019 - June 30, 2020

Principal Investigator

Areas of Expertise

Safety, Public Health, & Mobility Justice

Campus(es)

UCLA

Project Summary

Over 3,500 people have died on California’s streets and highways each year since 2016. Safety research has long demonstrated a correlation between speed and the severity of crashes. The basic rule for setting motor vehicle speed limits in California, and across the U.S. is the so-called “85th Percentile Rule,” which involves measuring a sample of speeds on a given roadway link and setting the speed limit below the speed traveled by the 15 percent of the fastest speeds measure, and above the 84 percent of the slowest speeds measured. This rule is deeply ingrained, both practically and legally in transportation engineering practice, but is now being scrutinized by those committed to improving traffic safety. This project will review the history and evolution of the 85th percentile rule in traffic engineering practice, and critically analyze and summarize research to date on its effects. The resulting report, policy brief, and Infographic will provide a complete and succinct summary of how the rule evolved into practice, and what researchers have found about its effects.