Portland's next on the list for variable speed limit signs

Portland's next on the list for variable speed limit signs
Traffic bottles up on Interstate 5 near the Rose Quarter on Thursday, Dec. 29, 2011 (photo by Shannon L. Cheesman, KATU.com Producer/Reporter).

PORTLAND, Ore. - Portland is about to become the state's next testing ground for variable speed limit signs.

It's all part of a pilot project to introduce the technology to Oregon. The basic idea behind variable speed limit signs is to monitor traffic in a particular area using sensors and then adjust the speed limit accordingly.

The signs are used in several other states but were first introduced in Oregon earlier this year on Highway 26 where it meets with Highway 47 (known as Staley's Junction).

That particular area had become a problem for drivers trying to turn onto Highway 26 from Highway 47, especially during the busy summer months when folks head to and from the coast. Variable speed limit signs now automatically turn on when sensors detect that traffic is heavy.

Now that the signs have been up and running in a rural setting for a while, it's time to try them out in an urban environment.

So in the next couple of months, the Oregon Department of Transportation (ODOT) will be working on installing a similar system in close-in Portland, specifically along Interstate 5 and Interstate 405. The graphic below provided by ODOT shows the proposed locations for the new signs.

While improving traffic flow will be the main function of the new system, reducing the number of crashes in congested areas is one of ODOT's main goals. The problem is drivers who come up unexpectedly on slow traffic and slam on the brakes but can't stop in time. The idea will be to slow them down enough in advance so it's not a problem.

"It's easier to stop your vehicle at 35 miles per hour than at 50 miles per hour," said ODOT spokeswoman Kimberly Dinwiddie.

When Is This Happening?

You won't see any signs along the freeway just yet. Right now ODOT crews are working on installing the sensors - some will be embedded in the roadway and others will be above ground.

Once the sensors are in place, ODOT will then spend time analyzing data and determining how best to use the system to help with traffic control.

The final step will be to install the signs and then turn them on. There is no firm date on when that will happen but ODOT is shooting for this spring.

Will It Really Work?

One question that might come to mind is how slowing down traffic even more can ease congestion. Everyone's already going slow as it is, right? Won't this just make it worse?

Dinwiddie said while it might sound counterintuitive, it actually makes sense when you look at it from a more scientific angle.

"It's like pouring rice into a funnel," she said. "If you pour the rice really fast, it'll get clogged up but if you pour it slow, it'll flow smoothly."

For a visual understanding of how it will all work, you can check out the Washington State Department of Transportation's video simulation below. Last year, Washington state instituted a similar program called Smarter Highways that uses the same type of technology. ODOT is looking closely at that model as they get ready to bring variable speed limit signs to Portland.


What's The Price Tag?

Oregon's variable speed limit sign project is being funded by $1.7 million in federal and state dollars. ODOT said compared to alternative ways of dealing with some of Portland's traffic issues, like adding more freeway space, it's cost effective.

"We don't have the money to expand the freeways, so we're using this technology," Dinwiddie said, adding that "we've really been doing this since the 80s when we started using ramp meters, so this is really just an extension of that."