Smarter Highways variable speed limits

Smarter Highways with variable speed limits

Helping drivers travel safer and smarter

Just like the symbols are warning you about lanes closed or blocked ahead, the speeds displayed on the overhead electronic signs are based on what’s happening on the road ahead. Congestion on the highway ebbs and flows and the variable speed limit signs reflect that fluctuation. It’s just like a driver speeding up and slowing down to adjust to the traffic flow. The only difference is that the Smarter Highways signs let you know in advance that the speed change is coming.


Ideally, approaching traffic will slow down and pass through the problem area at a slower but more consistent speed reducing stop and go traffic. By reducing stop and go traffic we’re also reducing the probability of an accident by giving drivers more time to react to changing road conditions. This helps drivers avoid the need to brake sharply as they approach congestion.


How do the signs know what speed to display?

Traffic sensors along the roadway collect vehicle speeds, congestion information and traffic flow rates. This information is continuously relayed back to the WSDOT’s Traffic Management Center in Shoreline and analyzed by computers. When circumstances that would benefit from lowered speed limits—like congestion—are identified, the computer reduces speeds incrementally to gradually reduce the approaching flow of traffic to the congested area.


Why do the signs display 40 mph when traffic is moving slower than that?

If traffic is stopped, the signs will display the system’s lowest speed, 40 mph. The signs won’t show a lower speed limit, even if traffic is moving slower. Think of the signs like a regular static speed limit sign. The speed limit is just that—a limit. It’s the maximum speed you should travel, however you might have to drive slower than the posted speed limit due to traffic or weather. Also, just like a static sign the speed limits are enforceable and you could be ticketed by law enforcement for exceeding the posted speed limit.



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