The Weird Way US Cities Set Speed Limits — and Why it’s Changing

In the United States, the long-held ‘85% rule’ — based in part on traffic studies conducted on rural roads in the 1930s and 40s — established speed limits based on the current speed distribution of free-flowing vehicles on a road.

As a blog post from the Oakland Transportation Management Association explains, the “the 85th percentile speed is the speed at or below which 85% of drivers will operate with open roads and favorable conditions.” Founded on the belief that drivers will drive at the speed, the rule assumes that anything above the 85th percentile would  create unsafe conditions.

But as an Associated Press article by Jeff McMurray points out, this creates a feedback loop that can cause officials to raise speed limits when drivers speed. Another approach, dubbed “City Limits,” calls for setting speed limits based on multiple factors that include a street’s level of activity and the likelihood of conflicts.

In its most recent update to the Manual on Uniform Traffic Control Devices (MUTCD), the Federal Highway Administration (FHA) acknowledged that the rule is not technically a rule, and that it has likely held too much weight in transportation planning  and that cities should consider safety and pedestrian use as well. Now, more cities, including Madison, Wisconsin and Seattle are lowering speed limits on roads with high pedestrian activity and a high rate of crashes.