Multimillion-dollar speed limit signs could better ease congestion if speeds enforced, engineer says

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The lead engineer behind multimillion dollar speed signs near Seattle says they're not being used to capacity.

He says congestion could be eased if there was more enforcement of the speed limits.

The digital signs that hang above I-5, I-90 and SR 520 were installed by the Washington State Department of Transportation in 2010 for about $42 million.


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Built to help drivers slow down for upcoming congestion and to avoid crashes, WSDOT says the variable speed limits have helped reduce collisions.

“By actually slowing down the speeds just before that congestion breaks down, you can increase the volume or capacity to the freeway by slowing down the speeds and have them a little closer on headways,” says Eric Shimizu, the lead engineer who designed the system.

KIRO 7 talked to Eric Shimizu, who designed it as the the second of its kind in the US, after Minneapolis, and modeled after those in Europe.

Shimizu believes if law enforcement more actively enforced the speed limits by ticketing those who went faster like Europe does with their system, when traffic is moving, congestion could be eased.

“If they could implement the enforcement by video detection like the Europeans have, you get a much higher obedience on the actual speed limits, Shimizu says.  

"There's vertical challenges or horizontal challenges where you can’t really see over a hill, and so the intent is to slow people down in those areas, so there won’t be as many secondary accidents or rear-ends  because they couldn't slow down in time,” said Shimizu.

The state patrol does not keep statistics for infractions on the variable signs, but we did pull speed ticketing for east and westbound I-90 for mile posts between the signs for the past few years.

In 2014, there were 3,600 speeding tickets. Last year, only 2,600.

If there is a spot where the signs are extremely efficient, Shimizu says it’s from reducing collisions by alerting drivers of incidents ahead.

“They measure upstream to see how the traffic is doing so they can communicate for the traffic coming to that point -- hey slow down, something is happening,” Shimizu said.

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