Target Zero Manager Doug Dahl discusses how speed limits are determined

Doug Dahl
The Wise Drive

Q: It’s obvious that many drivers routinely exceed the speed limit in many areas, not just on freeways. So the next logical question is: How are speed limits determined? One would hope that there is some science, not just politics or customary historical norms, that informs the speed limit decision for a stretch of road.

A: Your hope is not unfounded. The law does set default speed limits for various kinds of roads, which I guess you could say is setting limits based on historical norms, and they’re set in state statute by elected officials, so you could call that politics, but there’s more to it than that.

We don’t build roads however we want and then hope the default speed limit works for them. There are design standards (lots of them) that apply depending on the size and speed of a road. Let’s say that a city road engineer is designing a new road. State law sets the maximum limit for a city road at 25 mph. If that’s what the city needs, the engineer would design a road that meets the standards for that speed limit. But maybe the city has in mind an arterial with higher speeds. The law allows increases and decreases in speed limits “on the basis of an engineering and traffic investigation.” If the road is designed for it, the city could have a higher speed road. Apologies to the road engineers for oversimplifying here, but that’s my layman’s summary.

A speed limit could also change after a road is built. As homes, businesses, and driveways increase along a stretch of road, a city might do a study and decide to reduce the speed limit based on factors that weren’t present when the road was built. The right speed limit is based on more than the design of the pavement; it also includes the surrounding conditions. For example, a road in proximity with lots of pedestrians should have a lower speed limit than a similar road where there’s rarely a walker or bike rider.

There is an exception to the “engineering and traffic investigation” requirement for reducing a speed limit. The law allows local authorities to set 20 mph speed limits on non-arterial roads without a study. You’ll most likely find this applied in neighborhoods.

But setting an appropriate speed limit doesn’t guarantee that it’ll be followed. You’re correct that many people speed, and not just on freeways. Until recently, we didn’t have the hard data to support your observation, but in 2022 the Washington Traffic Safety Commission performed a speed observation survey. It’s just what it sounds like; measure the speed of cars as they drive by and count how many are speeding. The survey covered 206 locations in 30 counties around the state.

The bad news: 76 percent of drivers speed. But it’s also true that not all speeding is equal. Driving two mph over the speed limit is not the same as driving 15 mph over, and researchers have calculated the impact. For every one percent increase in speed there is a four percent increase in traffic fatalities. In Washington, over half of speeding drivers are between one and five mph over the speed limit. While ideally all drivers would respect the speed limit, it’s a small percentage of high-risk speeding drivers causing the greatest harm.

Nearly a third of all traffic fatalities in Washington involve a speeding driver. That’s 270 people in 2023. We could save a lot of lives just by slowing down. This is a call to be better drivers, at least until the robot cars take over.

The Wise Drive is hosted by Doug Dahl, a Target Zero manager for the Washington Traffic Safety Commission.