Eighty-Five Percent Solution: Historical Look at Crowdsourcing Speed Limits and the Question of Safety

Abstract

The “85th percentile rule” is commonly used to set speed limits in jurisdictions across the U.S. Modern interpretations of the rule are that it satisfies key conditions needed for safe roadways: it sets speed limits deemed reasonable to the typical, prudent driver, reduces the problematic variance in travel speeds among vehicles, and allows law enforcement to focus on speeding outliers. Authoritative publications regularly assert that the rule came about because early driving surveys often found that drivers moving at or below the 85th percentile of a speed on a given roadway were within one standard deviation of the mean speed for that roadway and were in the low involvement group for traffic incidents. But does this widely used rule for setting speed limits really have such a scientific pedigree? Given debates in cities around the U.S. about competing uses of street space, we examine where this rule of driver-set speed limits actually came from and whether rule developers’ rationales still hold true today. While most observers trace the rule to safety research and a 1964 report, we find that the 85th percentile rule actually emerged decades earlier amidst the nascent traffic engineering profession’s preoccupation with “traffic service” to increase vehicular throughput; and with respect to safety, the rule was explicitly intended as a starting point in speed limit setting, and not the last word.