States question decades-old method for setting speed limits

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 anda busy park.

- By Jerr McMuRRAY

PAUL SANCYA Associated Press ROSE HAMMOND has spent years urging officials to reconsider the speed limit on a busy Ohio road.

“What are you waiting for, somebody to get killed?” the 85-year-old chided officials in northwest Ohio, complaining that nothing was being done about the motorcycles that race by almost daily.

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 5 mph too low.

The reason dates back to studies on rural roads from the 1930s and 1940s that still play an outsize role in the way speed limits are set across the U.S.

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 5 mph 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.

Drivers have been setting the speed

The concept assumes that a road’s safest speed is the one most vehicles travel — neither too high nor too low. 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 putit.

“The problem with this approach is it creates this feedback loop,” said Jenny O'Connell, director of member programs for the National Assn. of City Transportation Officials. “People speed, and then the speed limits will be ratcheted up to match that speed.”