Commuters not heeding high tech speed limit signs

5:42 AM, Oct 3, 2013   |    comments
Smart Lane signs in downtown Minneapolis
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MINNEAPOLIS -- Researchers say Twin Cities commuters are contributing to traffic jams on I-35W, by ignoring high tech advisory speed limit signs designed to ease congestion.

That is among the findings of a comprehensive study of congestion along the I-35W corridor, by the Intelligent Transportation Systems Institute headed by a group of University of Minnesota engineers.

The signs are part of the smart lane technology built into a 16-mile stretch of the interstate, from Burnsville to Minneapolis. They were installed over the past three years with the use of federal anti-congestion grants.

"The hope is if we can give good information and people can make good choices that can ultimately improve the system," Brian Kary, the Twin Cities traffic operations engineer for the Minnesota Dept. of Transportation told KARE.

"If we can warn people of that congestion up ahead it doesn't catch people off guard, and we get fewer rear-end collisions at the tail of the queue."

Kary said the primary purpose of the smart signs is to get motorists to change lanes far in advance of an accident.  And that's why the most important symbol that appears on those rectangular overhead signs is the red X, which signals a lane is closed.

He said positive changes have been detected from his vantage point at the Regional Transportation Management Center in Roseville, and by first responders at traffic incidents.

"We certainly see some people moving over, and troopers are starting to see people moving out of the blocked lane more quickly than they used to," Kary explained.

But when it comes to getting people to slow down before they absolutely need to, which is the function of the advisory speed limit sign, those electronic displays aren't as effective as some hoped.

The idea is that if commuters work as a group to voluntarily reduce their speed, the cars will be more evenly spaced rather than getting knotted up in choke points. But the idea of slowing down to get some place more quickly isn't easy to grasp, and it requires drivers to act collectively.

Kary concedes it will take more time to change that behavior, to get people to respond to the variable speed limit suggestions. He noted that human nature leads drivers to charge ahead as fast as they can rather than slowing down before they're forced to by stalled traffic.

"It's still a little bit of an experimentation. This is really only one of two systems like this in the country, the other one being in Seattle."

In parts of Europe the variable speed limits are more than just a suggestion. They're enforced just like a regular speed limit sign.

In Minnesota and Seattle, on the other hand, the smart lane signs are referred to as "cautionary signs." You won't be ticketed for ignoring them and driving the posted speed.

"They're just purely advisory. They kind of advise you there's slowdowns up ahead," Kary said.

"But in Europe they don't have the ramp metering that we use here, and that allows us to squeeze more capacity out of our existing lanes without the need to enforce those variable speed limits."

MnDOT used state funds to add the smart lane signs on an 8 mile stretch of I-94 between downtown St. Paul and downtown Minneapolis. That was crucial, according to Kary, because MnDOT had to sacrifice shoulders in part of that corridor, to create new lanes.

"If you look at I-94 between 35W and 280, there's essentially no shoulder in that area," he said.

"So to mitigate for the fact there's no shoulder we're putting up these signs to help manage those incidents."

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