Effects of lowering speed limits on crash severity in Seattle

Hu, Wen / Cicchino, Jessica B.
Insurance Institute for Highway Safety
March 2023

Introduction: Effective November 2016, the default speed limit in Seattle was lowered from 25 to 20 mph on nonarterial streets and from 30 to 25 mph on arterial streets, unless otherwise posted. In the downtown area, signs indicating the new speed limit were installed on arterials when the lower default limit took effect. Outside the downtown, new speed limit signs were installed on some arterials starting in 2018. The study evaluated effects of the speed limit reduction on crash severity in Seattle.
Method: Police-reported crashes in Seattle and three control cities in Washington before and after the speed limit change were examined. Logistic regression analyses evaluated effects of the speed limit reduction on odds that a crash involved a fatal (K), disabling (A), or evident (B) injury inside and outside Seattle’s downtown. Separate analyses were performed for all crashes except those occurring on interstates and freeways, crashes on arterials, and crashes on nonarterial roads.
Results: The speed limit reduction was associated with a significant 17.2% reduction in odds of a crash involving KAB injury among all crashes and a 19.9% reduction for crashes on arterials in downtown Seattle. There were smaller reductions outside the downtown (7.4% for all crashes and 10.7% for crashes on arterials), but they were not significant.
Conclusions: Communities should consider lowering speed limits to improve safety for all road users. When doing so, they should not wait too long to modify speed limit signs to remind drivers of the new speed limits to maximize the safety benefits.