Lowering speed limits makes Seattle streets safer

Crashes on Seattle streets were less likely to cause injuries after the city lowered speed limits, a new study by the Insurance Institute for Highway Safety shows.

“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.”

In downtown Seattle, lowering default speed limits reduced the likelihood that a crash would involve an injury by a fifth on arterial roads, IIHS researchers found. Outside of the city center, where the new limits were less consistently implemented and publicized, there were smaller injury reductions, but these were not statistically significant.

An earlier IIHS study showed that lowering speed limits in Boston resulted in less speeding, but it didn’t investigate the impact on crash or injury rates. The new study of Seattle’s program is one of the first to examine the effect of lower limits on injury rates in a large, populous U.S. city.

“Everybody thinks of highways when we talk about speed limits, but reducing speeds on city thoroughfares and residential streets is just as important,” Harkey said. “Nationwide, nearly a third of crash deaths occurred on urban arterials in 2020.”

Higher speeds make crashes more likely by reducing the time a driver has to react and increasing the distance required to stop the vehicle. Higher speeds also increase the energy involved in a crash, raising the odds of an injury.

As of November 2016, Seattle lowered the default speed limits from 30 mph to 25 mph on its arterial roads and from 25 mph to 20 mph on smaller, mostly residential streets, unless otherwise posted. To make drivers aware of the change, the city conducted a public outreach campaign and installed gateway signs indicating the new citywide limits on arterial roads into the city, highway off-ramps and ferry terminals.

At the same time, new 25 mph speed limit signs were installed on arterial roads within the downtown area. Most arterials outside the city center had higher posted speed limits, and these mostly remained in effect until 2018, when Seattle began installing new signs on more arterials outside the downtown. By the end of 2019, speed limit signs had been changed in eight of the city’s 32 urban centers and villages, including the downtown, and on some arterial corridors in other areas.

IIHS surmised that the largest impact would be a reduction in crash injuries — which is the main focus of the Safe System approach. Researchers looked at the proportion of crashes involving fatal, serious or evident injuries, comparing the before-and-after injury rates with three control cities in Washington where there were no known changes to speed limits over the study period.

In all four cities, a large majority of crashes occurred on arterials. The proportion of crashes with injuries dropped on these major roads in the three years after the speed limit reductions in the downtown area of Seattle, while the proportion of crashes involving injuries increased in the control cities. This was also true for all arterial and smaller roads in the downtown areas combined.

Controlling for weather, lighting conditions and other factors, Seattle’s speed limit reduction was associated with a statistically significant 17 percent drop in the odds of an injury crash downtown and a nonsignificant 7 percent drop outside the city center. On arterial roads only, there was a statistically significant 20 percent reduction in the odds of an injury crash downtown and a nonsignificant 11 percent decrease outside the city center.

After the period covered by this study, in 2020, Seattle lowered speed limits on most remaining arterial roads to 25 mph. At the same time, it removed about 750 old signs and installed nearly 2,600 new ones in greater density.

“These results suggest that communities can reap substantial benefits by lowering speed limits,” said IIHS Senior Research Transportation Engineer Wen Hu, the lead author of the study. “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.”

On nonarterials, there was no significant change in the odds of an injury crash, either downtown or in other parts of the city. However, that might partly be due to the small sample size and the nature of the roads themselves. Many residential streets in Seattle are narrow lanes with on-street parking on both sides, so drivers may not be comfortable driving much faster than 20 mph regardless of the default limit.

It's also likely that the program was more effective where speed limit signs were posted than in areas where drivers were expected to remember that the default limits had changed throughout the city. That explains why the largest effect was observed downtown, where new speed limit signs were put in place at the same time the limits were lowered.

“When it comes to reminders, a clearly visible speed limit sign is pretty hard to beat,” said Hu. “We expect Seattle to see even bigger benefits as the city completes the transition to new signage.”