85th percentile speed limits: “Fast, Furious & Fatal”

By Eric Sundquist

In 2017, when the City of Los Angeles reviewed operating speeds across its system, it ended up raising speed limits on 89 miles of roadway. Why would a city with a Vision Zero goal raise speed limits in corridors with a history of crashes? Because of a California statute that locks in the discredited practice of setting speeds at the 85th percentile.

The LA story is described in a UCLA policy brief that doesn’t seem to be online, as well as a longer paper by Ribeka Toda. The 2017 speed study revealed faster operating speeds in many corridors, which led to the city’s action to raise speed limits. It lowered limits on 52 miles of streets.

As others have found, Toda argues that the 85th percentile is not a good standard of safety. She cites an “injury minimization” scheme (Table 1), and recommends that local governments be freed from the current standard in order to pursue these safer limits.

Condition Recommended speed limit (mph)
Mix of motorized and unprotected users 20
Roads with access where side-impact crashes can result 30
Undivided roads where head-on crashes can result 45
Controlled access, median separated roads 60 or higher
Table 1. Injury minimization limits. Original source: FHWA.

Toda concludes: “Other states have found ways to increase speed limit flexibility for local jurisdictions, including statewide maximum speed limits for urban areas, authorizing local jurisdictions to set speed limits on their streets, and employing pilot projects to test alternative methods for setting speed limits in cities.”

Eric Sundquist is Director of SSTI.