The Origins of Speed Limits

Motor vehicle crashes claim over 30,000 lives per year, with related costs in the hundreds of billions. While we sometimes view that frightening statistic as inevitable, there are reasons to reexamine speed limits and how we set them.

August 2, 2015, 9:00 AM PDT

By Philip Rojc @PhilipRojc


Speed Limit

Jon Bilous / Shutterstock

In this article, Anna Maria Barry-Jester asks the question: where do speed limits come from? "Given the social and economic toll of speeding, one might assume that we set speed limits with careful calculations aimed at maximizing safety." However, she continues, "in most places, speed limits are largely determined by the speed most people feel safe traveling."

This average comfortable speed sits at a hypothetical 85th percentile: given the road, 85% of drivers will cruise below it and 15 percent will speed above it. But road engineers know this, and build margins of "safety" into new roads: "[the 85th percentile is] used to design a road to meet that speed at a minimum, with a factor of safety allowing for faster travel."

The article discusses a historical shift from keeping roads safe from cars, to keeping them safe for cars. "City speed limits were set below 20 mph until automakers and industry groups realized that such low limits were going to hurt sales. To change public opinion, campaigns were started to criminalize pedestrians in the road (jaywalking) and to shift the blame for accidents from cars to 'reckless drivers.'"

Nowadays, people are less willing to accept the auto-centric status quo, at least in cities. Recent efforts like New York's Vision Zero plan mark departures from our "culture of acceptance" around traffic fatalities. 

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