Smarter Highways 102: Variable Speed Limits

 by guest blogger Annie Johnson

Update: A number of changes were made to the operations of the Smarter Highways signs in October 2011. In addition to decreasing the minimum speed to 30 mph, we are using additional upstream gantries to initiate the speed-reduction process earlier, using the larger Smarter Highways variable message signs to post travel times, and using the signs in a coordinated effort with the City of Seattle to communicate to drivers the possibility of using city streets instead of the freeway. 

With Smarter Highways live on northbound I-5 in south Seattle and activation on SR 520 just around the corner later this fall you’ve mastered the basics. You know that the yellow merge arrows are going to help you around the blocked lane and that a red “x” is telling you that a lane is closed. Now it’s time for what we like to call Smarter Highways 102: variable speed limits.

It’s about what’s ahead
Just like the symbols, the speeds displayed on the overhead electronic signs warn you about what’s happening on the road ahead of you. 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.

Speed limits reflect the conditions
The electronic speed limits displayed on the signs reflect travel conditions not necessarily how fast or slow traffic is moving in the freeway lanes. 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 just like a regular static speed limit sign. It’s the maximum speed you should travel, however as any driver knows 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.

Helping drivers travel safer and smarter
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.

So how do you come up with that speed?
In urban areas traffic sensors along the roadway collect vehicle speeds, congestion information and traffic flow rates. This information is continuously relayed back to our 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. This is different than the electronic speed limit signs you see on I-90 over Snoqualmie Pass and on US 2 over Stevens Pass. These signs are manually operated by our South Central and North Central region Traffic Management Centers to reflect the current road and weather conditions on the passes.

So there you go. That’s Smarter Highways 102. Any questions?