State law (RCW 46.61.400) sets Washington’s basic speed law and the maximum speed limits for state highways, county roads, and city streets.
The statute also authorizes agencies to raise or lower these maximum speed limits, when supported by an engineering and traffic investigation.
Most states have a basic speed law which recognizes that driving conditions and speeds may vary widely from time to time. No posted speed limit can adequately serve all driving conditions. Motorists must constantly adjust their driving behavior to fit the conditions they meet.
Speed limits encourage consistent travel speeds, fostering safety for the traveling public by reducing the speed differentials between motor vehicles.
Speed limits reflecting the speed most motorists naturally drive are selected in part by determining the “85th percentile speed” (the speed that 85 out of 100 vehicles travel at or below).
This method is based on the principle that reasonable drivers will consider roadway and roadside conditions when selecting travel speeds.
When setting speed limits, engineers also consider other factors like:
- Roadway characteristics, shoulder condition, grade, alignment and sight distance
- Roadside development and lighting
- Parking practices, e.g., angle parking, and pedestrian and bicycle activity
- Collision rates and traffic volume trends
- Right lane/entering traffic conflicts (for freeways)
The range of travel speeds is reduced when speed limits are set near the 85th percentile speed and adjusted for the other influencing factors.
If you are concerned about a speed limit, contact the appropriate public agency. For state highways contact WSDOT, and for city streets or county roads contact the agency’s public works or transportation department.
The agency may conduct the engineering and traffic investigation required by law to establish a speed zone, including speed studies, collision history, and an investigation of the other highway, traffic and roadside conditions.