Q: It’s obvious that many drivers routinely exceed the speed limit. So the 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 lots of design standards 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 that standard. 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.

A speed limit could also change after a road is built. As homes, businesses and driveways increase along a 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. Law allows local authorities to set 20 mph speed limits on non-arterial roads without a study. You’ll most likely find that applied in neighborhoods.

But setting an appropriate speed limit doesn’t guarantee that it’ll be followed. You’re correct that many people speed. In 2022 the Washington Traffic Safety Commission performed a survey. It measured the speed of cars as they drove by and counted how many were speeding. The survey covered 206 locations in 30 counties in the state. The bad news: 76% of drivers speed.

But it’s also true that not all speeding is equal. Driving 2 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 4% increase in traffic fatalities. In Washington, over half of speeding drivers are between 1 and 5 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 robot cars take over.

Doug Dahl writes a weekly column for this newspaper. He is with the state Traffic Safety Commission.