State says speed signs reduce crashes

September 3, 2014

By Ari Cetron

Chris Pribbernow, of Sammamish, finds himself commuting back and forth to Seattle regularly, but he, like other drivers, sometimes finds himself flaunting the law as he does it.

The state installed variable speed signs along Interstate 90 and state Route 520, and when there’s congestion ahead, the signs will drop the speed limit for the road below the standard 60 mph.

When that happens, Pribbernow said he, and most of the other drivers, just keep zipping along the freeway.

“I ignore the speed signs and continue to go with the flow of traffic,” Pribbernow said. “I guess I just don’t take them as seriously.

The state installed the first set of those signs, formally called Active Traffic Management signs, along I-5 in 2010, said Travis Phelps, Washington State Department of Transportation spokesman.

The signs work automatically, with a computer checking traffic volumes and generating automatic slowdown messages, Phelps said. He was quick to note that the center is staffed round-the-clock, and human operators can, and do, override the machine.

While they’ve been on the passes for years, the state is now trying to use them to manage traffic in more congested areas, Phelps said.

And while some people may ignore the warnings, they seem to help reduce car crashes.

Why weekends?

State officials were a bit surprised at the dramatic reduction in accidents on weekends, which they attribute to the variable speed signs.

Transportation engineers have a theory that weekend drivers are more likely to be taking unfamiliar routes. While many people know what to expect during their daily commute, weekend drivers may be on roads to which they are not accustomed.

Matt Beaulieu, a traffic engineer with WSDOT, theorizes that by using the signs to give people more information further in advance, they allow people more time to process that information and use it to make a more gradual change on an unfamiliar road.

“We’re trying to avoid than panicked breaking,” he said.

Phelps said the idea is to slow traffic down ahead of a slowdown further along the road. That can lead to fewer drivers slamming on the breaks and causing crashes as they approach the area.

“We’re trying to get as much information to the driver as soon as possible,” Phelps said.

Anecdotally, at least some of the drivers don’t seem to heed the warnings. Pribbernow, and others interviewed, say they don’t notice cars actually slowing down when the speed signs change.

Drivers often just continue to go with the flow of traffic as long as there’s an open road and then slow down when they must.

“I don’t want to be that guy everyone is passing and honking at,” Pribbernow said.

The state patrol doesn’t track how many ticket they write to people who violate the speed signs, said Trooper Karim Boukabou, but he noted it is something they can enforce.

While it might be difficult in some cases, where a speed limit might change after a driver has passed a sign, Boukabou said more egregious cases are the sort of thing where he might pull someone over, like observing someone driving 60 mph under a sign that says 40.

“I might have a chat with that person,” Boukabou said.

More often, he said, the signs become moot. Often, when the sign reads something like 40 or even 30 mph, traffic is at a standstill and drivers can barely crack double digits.

Boukabou said at that point, troopers would likely turn their attention to other offences, such as HOV cheaters, or distracted driving like someone using a cell phone or texting while they drive.

In general, Boukabou said the signs can help. The lower numbers can help warn drivers that a slowdown is coming and help to forestall secondary accidents.

“It’s a great tool for advisory,” he said.


Accidents down

State data points to the signs working to reduce the number of accidents.

Besides changing speed limits, the signs can alert drivers to a closed lane ahead due to a crash or construction zone. That gives drivers more time to adjust and means fewer motorists zipping up close to the scene before moving over, said Morgan Balogh, a traffic engineer with WSDOT.

“Our folks, and people who work on the road, really appreciate the signs,” he said.

A WSDOT study also points to the signs as reducing accidents.

The study looked at the number of collisions for three years before the signs were installed and compared them to a three year span after. It showed a reduction in accidents by 3.5 percent to 7 percent on weekdays and 13 percent to 20 percent on weekends (see sidebar).

The study applied to the signs only on I-5 northbound, heading into Seattle.

Matt Beaulieu, also a WSDOT traffic engineer, explained the state’s methodology in the study. He noted that factors such as traffic volumes, which would fluctuate during those time spans, would make it difficult to compare the section of road to itself before and after the signs were installed.

Instead, Beaulieu explained, the state compared I-5 northbound heading into Seattle to I-5 southbound heading into Seattle. In theory, more global factors, such as a recession-induced reduction in traffic, would be roughly the same on both of those sections of highway.

The only major difference between those sections, Beaulieu said, is the signs having been installed on the northbound lanes heading into Seattle.

The study found that accidents were up on the southbound segment, and indeed on other segments of highway in King County that WSDOT studied. However, they decreased on the segment of I-5 where the signs had been installed.

Beaulieu said that WSDOT has not been able to study the effects of the signs on I-90 and state Route 520. Neither of those roads has a corresponding section of road that is not signed, making it difficult to draw conclusions about changes there.

“Those factors really made it hard to look at what was going on,” he said.

He also said the state can’t really compare its results to that of other jurisdictions with the signs. Usually, Beaulieu said, the signs go in a part of a larger traffic management plan, such as installing metered onramps, or having incident management in place.

By placing in multiple techniques at once, it makes it hard to attribute a change to any one of them.

In this area, we already had those in use before adding the variable signs.

“It’s rare to put in this equipment into a fairly mature system,” Beaulieu said.

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