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There's a Long-Held Way to Set Speed Limits, and It's Under Fire

States are rethinking the '85% rule'
By Newser Editors and Wire Services
Posted Aug 18, 2025 6:54 PM CDT
States Rethink How to Set Speed Limits
A vehicle drives on Mitchaw Road past the Lakes of Sylvania senior living community on Thursday, Aug. 7, 2025, in Sylvania Township, Ohio.   (AP Photo/Paul Sancya)

Rose Hammond pushed authorities for years to lower the 55mph speed limit on a two-lane road that passes her assisted living community, a church, two schools, and a busy park. "What are you waiting for, somebody to get killed?" the 85-year-old chided officials in northwest Ohio. Amid growing public pressure, Sylvania Township asked county engineers in March to analyze whether Mitchaw Road's posted speed is too high. The surprising answer: Technically, it's 5mph too low. As the AP reports, the reason dates back to studies on rural roads from the 1930s and 1940s that still play an outsized role in the way speed limits are set across the US—even in urban areas.

Born from that research was a widely accepted concept known as the 85% rule, which suggests a road's posted speed should be tied to the 15th-fastest vehicle out of every 100 traveling it in free-flowing traffic, rounded to the nearest 5mph increment. But after decades of closely following the rule, some states—with a nudge from the federal government—are seeking to modify if not replace it when setting guidelines for how local engineers should decide what speed limit to post. The concept assumes that a road's safest speed is the one most vehicles travel. If drivers think the speed limit should be raised, they can simply step on the gas and "vote with their feet," as an old brochure from the Institute of Transportation Engineers once put it.

"The problem with this approach is it creates this feedback loop," said Jenny O'Connell of the National Association of City Transportation Officials. "People speed, and then the speed limits will be ratcheted up to match that speed." The association developed an alternative to the 85% rule known as "City Limits," which aims to minimize the risk of injuries by setting the speed limit based on a formula that factors in a street's activity level and the likelihood of conflicts, such as collisions. The report points out the 85% rule is based on dated research and that "these historic roads are a far cry from the vibrant streets and arterials that typify city streets today."

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Amid a recent spike in road deaths, the Federal Highway Administration sent a message to states that the 85% rule isn't actually a rule at all. In its first update since 2009 to a manual that establishes national guidelines for traffic signs, the agency clarified that communities should also consider such things as how the road is used, the risk to pedestrians, and the frequency of crashes. Leah Shahum, who directs the Vision Zero Network, a nonprofit advocating for street safety, said she wishes the manual had gone further. "The 85th percentile should not be the Holy Grail or the Bible, and yet over and over again it is accepted as that," Shahum said. Much more here.

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