Question: Who sets speed limits and how are they decided?
Answer: Should we start with the science or the politics? Let’s start with politics, since that gets us on track to answer the first part of the question. Because speed limits are enforceable laws, they are set by elected officials. At the state level, RCW 46.61.400 defines the speed limits for city streets at 25 mph, county roads at 50 mph and freeways at 60 mph. However, the next few sections of the RCW specify situations where state and local officials can adjust speed limits as appropriate.
In Whatcom County, for example, our statutory speed on county roads is 35 mph. For our local roads, the elected officials with authority to change speed limits are the members of a city or county council, which, if I recall correctly from my high school civics class, make up the legislative branch of our local government. I just point this out for the folks who feel the need to criticize a speed limit during the issuance of a speeding ticket. The police are part of the executive branch: They enforce the laws, but they don’t get to make them. As part of issuing a ticket, the officer will give the offender an opportunity to have a day in court, our judicial branch, and - ta da! — we’ve included all three branches of government in this article.
How do they decide limits?
But back to the question. Our elected officials set the speed limits, but how do they decide what is appropriate? A city or county council will look to input from its citizens and expertise from engineers. Let’s follow a common path to a potential speed limit evaluation. A request for a speed limit change may come as a letter from a citizen to the local city or county council. A council member may then talk with the jurisdiction’s traffic engineer to assess the speed limit for that road. Often people who get tickets on a given street think the speed limit is too slow, and people that live on that street think the speed limit is too fast. How does a traffic engineer determine who is right?
A primary principle of traffic engineering is to provide safe and quick-as-possible travel between destinations. The engineer has to balance those two objectives. This is often done through a speed study. A speed study consists of a review of the function, design and actual use of the road. Is the road intended for heavy commuter traffic or light local traffic? How wide are the lanes? How wide are the shoulders? Is it straight or curvy? Are there a lot of intersections? These and many other questions about the road get answered in the study.
Then the engineer conducts a traffic count to determine the actual quantity of traffic and how fast the vehicles drive. This data provides the “85th percentile speed.” That term just means the speed at which 85 percent of the cars travel. On a well-designed road with an appropriate speed limit and effective enforcement, the 85th percentile speed should be pretty close to the posted speed limit.
The engineer also researches the number and rate of collisions on the road and compares them to the collision rate for the overall jurisdiction. Collision rates are the number of crashes per million miles traveled.
With all this research completed, the traffic engineer evaluates it and makes a recommendation. If the crash rates are high and the 85th percentile speed is at or near the posted speed limit, the engineer likely would recommend reducing the speed limit. If the crash rate is high but the 85th percentile speed is significantly higher than the posted speed limit, the recommendation might be for additional enforcement of the current speed limit. If the crash rate is low and the 85th percentile speed is much higher than the posted speed limit the engineer could recommend increasing the speed limit.
If any traffic engineers are reading this, they’re probably thinking I’ve grossly over-simplified how they make recommendations based on a traffic study, and they’re right. As you can imagine, many more factors than the few I’ve described here influence the final recommendation. But hopefully this is helpful in understanding the speed study process.
Once the traffic engineer presents the results of the study to the city or county council, the council evaluates the report, along with public input. If the desire of a citizen or group of citizens is in conflict with the recommendations of the traffic study, the council has to weigh the input and make a decision. This brings us back to where we started, with science and politics. Ultimately, both the engineering research and the will of the people determine the speed limits for our roads.
Road Rules is a regular column on road laws, safe driving habits and general police practices. Doug Dahl is the Target Zero Manager for the Whatcom County Traffic Safety Task Force. Target Zero is Washington’s vision to reduce traffic fatalities and serious injuries to zero by 2030. For more traffic safety information visit TheWiseDrive.com. Ask a question.