While it’s too soon to tell if this signifies a trend, a number of jurisdictions around the country are lowering speed limits on some of their roadways.
In July, Bozeman, Montana passed an ordinance that recommends lowering speed limits on city-controlled arterials. In September, the City of Oakland, California announced plans to lower the speed limits in a dozen business activity districts. The Lubbock City Council in November voted to reduce speeds on five segments of major Lubbock thoroughfares by 5 to 10 mph. That same month, Falls Church, Virginia introduced a 20 mph speed limit on many residential streets. And a Honolulu, Hawaii city councilmember is pushing to reduce the default residential speed limitfrom 25 mph to only 15 mph.
Colorado DOT engineers are also changing their philosophy regarding speed limits, embracing a new technique that allows state traffic engineers to weigh certain factors more heavily while setting speed limits on state-controlled roads and highways. Those include the road’s purpose, geometry and the number of pedestrians and cyclists who use it.
Historically, most state DOTs use the 85th percentile rule when determining speed limits. This technique involves monitoring a roadway and identifying the speed at or below which 85% of the drivers are traveling, and then setting the speed limit by rounding from that speed to the nearest 5 mph marker. This method is tied to current driving tendencies and the design of a road, and often results in high speed limits regardless of whether that aligns with an agency’s stated safety goals.
Higher driver speeds clearly lead to more dangerous roads, especially for pedestrians. Research shows that a person hit by a car traveling at 35 mph is five times more likely to die than a person hit by a car traveling at 20 mph. Higher speeds are known to make crashes more likely because they reduce the time a driver has to react, increase the distance required to stop the vehicle, and increase the energy involved in a crash, thereby raising the odds of an injury.
But does lowering a speed limit automatically translate into slower speeds? Speed limits are only one factor influencing how fast people will actually drive, especially when roadways are designed for travel at higher speeds, which is often the case.
In 2014, the Washington State Legislature enacted the Neighborhood Safe Streets Bill, which allows cities to lower speed limits to 20 mph on residential streets and non-business arterial streets without having to go through expensive traffic studies (Feet First was part of a coalition of groups supporting this bill). The City of Seattle soon after took advantage of this law and lowered the default speed limit on roads by 5 mph in many parts of the city – from 30 mph to 25 mph on its arterial roads and from 25 mph to 20 mph on smaller, mostly residential streets.
A study released last spring by the Insurance Institute for Highway Safety, is one of the first to examine the effect of lower limits on injury rates in a large, populous U.S. city. It found there was a 17 % drop in the odds of an injury crash in Seattle’s city center, and a 20 % reduction in the likelihood of an injury crash downtown on arterial roads. “These results illustrate the value of rethinking speed limits,” David Harkey, president of the Insurance Institute, said in a statement. “Crashes still happened after Seattle’s changes, but they weren’t as dangerous.”
While this study provides support for the idea that lower speed limits can lead to safer streets, the physical design of a roadway is still a significant factor in how fast people drive, separate from the posted speed limit. Although Feet First advocates lowering speed limits, particularly on residential streets, these changes should be accompanied by physical traffic calming measures such as traffic circles and narrower travel lanes.