Who decides the speed limits on our roads?

This took a bit of research, but I’ve got some answers for you. I talked with a couple of engineers that work for Whatcom County and got the low down on who gets to decide how fast we can drive on the roads. As you might expect, speed limits decisions come from the combined input of government agencies. At the most fundamental level, the Revised Code of Washington (RCW) determines speed limits throughout the state. (Read it here: RCW 46.61.400.) To start with, the law requires motorists to drive reasonably and consider the conditions and potential hazards, regardless of what the speed limit signs say. The RCW sets the basic speed limits at 25 MPH on city and town streets, 50 MPH on county roads, and 60 MPH on state highways.

Given those standards, you may wonder why many roads don’t conform to it. That’s where RCW 46.61.415 comes in. This statute gives local authorities the ability to alter maximum speed limits. On a local level, engineers in public works assess existing speed limits based on the following factors.

• The 85th percentile speed of vehicles traveling on the road (I’ll explain later)
• Road characteristics, shoulder condition, grade, alignment, and sight distance
• The pace speed
• Roadside development and environment
• Parking practices and pedestrian activity
• Reported crash experience for at least a 12 month period
• Other factors such as route development or comprehensive plans

You can probably figure out what most of those factors mean, but a couple of them need explanation. At least, I needed them explained.

85th percentile: This means that engineers measure the speed of all vehicles on a road, and then calculate the speed at which 85% of us drive our cars at or below. As it turns out, roughly 85% of all motorists drive a safe speed on any given road, regardless of speed limits. Most drivers pay attention to their surroundings and continually adjust their vehicle rate to maintain a safe driving speed. 

Pace Speed: Engineers calculate the pace speed by determining what 10 MPH spread encompasses the greatest percentage of drivers on the road. For example, if a speed survey showed that 65% of all cars on a given road traveled between the speeds of 36 and 46 MPH, that road would have a pace speed of 36 MPH. While not as common a tool as the 85th percentile, pace speed gives a good indication of a safe rate of travel for a road.

By evaluating the above factors, engineers come up with a speed that they believe will fit the road. The engineers I spoke with pointed out that they cannot guarantee the complete safety of any road design, because most traffic crashes are caused by poor driving behavior. Apparently, even the greatest engineers still can’t compensate for a driver’s lack of common sense. Once the calculations are complete, they round up or down to the nearest 5 MPH increment and recommend the speed to the local government council.

The local governmental council has the final word on speed limits. Typically, they will accept the recommended speed from their engineers. However, sometimes they don’t. Here’s a possible example: A road in a community has a speed limit of 35 MPH and needs major upgrades. The road department determines that because of the volume of traffic, the road should be widened. In order to widen the road, the county has to get right-of-way from land owners along the road. The land owners agree to giving the county right-of-way in exchange for a guarantee that the speed limit remains the same because they don’t want their neighborhood turning into a racetrack. During the planning and building process, engineers evaluate a variety of factors and determine that the new road design would support 45 MPH. They submit this recommended speed limit to the council, but the council rejects it due to the commitment they made to the local community.

Another issue that affects speed limits is consistency. Whatcom County has widely varying geography, which means that one portion of a road my have great visibility, other portions may not. Hills and curves affect the safe speed of a road. If a road frequently changes from flat and straight to hilly and curvy, the speed limit on the road may get set at the safe speeds for the hills and curves. The engineers want to set safe speed limits, but they also do not want to confuse motorists by changing speed limits every few blocks.

On existing roads, speed limits get evaluated based primarily on collisions and citizen requests, with collisions being the greater factor of the two. In 80 – 90% of traffic surveys that come from citizen requests, the engineers conclude that despite the assurances of the residents in the community that the speed limit is way to fast, the existing speed limit fits the road best. 

The engineers I spoke with pointed out that reducing speed limits does not reduce speeding. If a road has an appropriate speed limit, most drivers will naturally obey it because, like we discovered with the 85^th percentile, safe motorists drive at a safe speed. Reducing the speed limit will tend to create violators out of safe motorists, while those who excessively sped before continue to do so. On well engineered roads with proper speed limits, the real solution to speeding problems comes from enforcement.