IIHS study: Seattle residents in fewer injury crashes after speed limits lowered

Most states employ low speed limits in residential areas to prevent collisions and pedestrian injuries, with limits typically between 25 and 35 mph. In 2016, Seattle began lowering some of its speed limits from 30 to 25 mph on arterial roads and 25 to 20 mph in neighborhoods. That may sound like a bummer to car enthusiasts, but the Insurance Institute for Highway Safety (IIHS) recently published its study on the changes, finding that drivers are less likely to be injured in a crash.

Lower speed limits had a significant effect on the likelihood that people would be injured in a crash. After the speed limits changed, the IIHS found that drivers had a 17 percent lower chance of injury in the city’s downtown area. Things were even better on arterial roads, where drivers had a 20 percent lower chance of being injured.

In an earlier study that looked at Seattle's first round of speed reductions, the IIHS compared three control cities in Washington state where limits weren't changed, vs. Seattle. By late 2019, Seattle had changed the speed limits in a quarter of its major neighborhoods. Most crashes happen on arterial roads in all of the cities, but Seattle saw a decline in crashes with injuries while control cities saw increases.

City officials took notice of the study and, in 2020, lowered most of the remaining speed limits on arterial roads to 25 mph. The effort involved removing hundreds of old speed signs and replacing them with 2,600 new ones. The new signs are more densely installed to ensure everyone knows about the change.

Though Seattle’s arterial roads saw improvement, there were no changes in the likelihood of injury during crashes on neighborhood streets and tight downtown streets. The IIHS points out that the narrow roads may naturally limit drivers’ comfort with speed and noted that it could be possible that areas with better signage saw more improvement.

“When we talk about the Safe System approach, we always stress that nobody should have to die because of a mistake,” said IIHS President David Harkey. “These results illustrate the value of rethinking speed limits. Crashes still happened after Seattle’s changes, but they weren’t as dangerous.”

“These results suggest that communities can reap substantial benefits by lowering speed limits,” said IIHS Senior Research Transportation Engineer Wen Hu, the study's lead author. “To reduce injuries even further, communities should combine lower speed limits with engineering solutions, public education about the importance of reduced speeds, and high-visibility enforcement.”