Rose Hammond pushed authorities for years to lower the 55 mph speed limit on a two-lane road that passes her assisted living community, a church, two schools and a busy park that hosts youth sports leagues.

"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 limit is too high .

The surprising answer: Technically, it's 5 mph too low.

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 U.S. — even in urban areas.

Born from that research was a widely accepted concept known as the 85% rule, which suggests a road's

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