NTSB Report: The 85th Percentile rule is killing us

Advocates have been asking DelDOT for years to lower the speed limit on Route 4 in Ogletown, from 50 mph to 40, or at least 45 for a start. Despite several traffic studies showing speeds as high as 57 mph even in front of area schools including the Delaware School for the Deaf, they insist that the rule is well justified.

From Streetsblog -- Traffic deaths in the U.S. are mounting, reaching more than 40,000 last year, and, according to a recent draft report by the National Transportation Safety Board, speed is the overlooked factor.

The NTSB reported that speeding accounts for about 10,000 deaths a year -- as many as drunk driving. One of the agency’s key recommendation was to change the way streets are designed by reforming the “85th percentile rule,” a laissez faire approach that seeks to accommodate motorist behavior instead of engineering streets for safety.

It’s an argument that Randy LoBasso at the Bicycle Coalition of Greater Philadelphia has been making for a long time. Now that the NTSB report is vindicating advocates’ critique of the 85th percentile rule, he writes:

"The 85th Percentile idea, based on the 1964 “Solomon Curve” says speed limits should be set at what 85 percent of drivers think is healthy. It was created back when the highway system was still young, cars didn’t approach speeds as quickly as they do today, and we didn’t have the sort of statistics and research on traffic dangers we do today. [More . . . ]

Despite numerous residential, school, and retail zones that are rich with bike-ped activity, the speed limit on Route 4 in Ogletown/S. Newark remains posted at 50 mph. The 85th Percentile basically assumes that 85% of people are good drivers, and with that, sets it to their average speed. It's complete nonsense, because far less than 85% are truly good drivers. But more significantly, it fails to consider the safety of pedestrians, bicyclists, and other vulnerable road users.