Speed limit to vary on stretch of I-5, in bid to ease jams

Next year, the speed limit on a six-mile stretch of Interstate 5 will change several times a day, in an attempt to make traffic flow more smoothly.

The switch is expected in late 2009 in the northbound lanes, from south Boeing Field to the Interstate 90 junction in Seattle, Carol Hunter, a project manager for the state Department of Transportation, said Monday.

Overhead electronic signs will dictate the speed limits, in a system called "speed harmonization" that's used on some European highways. Drivers near the airfield might be held to 30 or 40 mph, instead of going the standard 60 mph, only to meet stop-and-go traffic downtown.

There should be fewer rear-end crashes, Hunter said.

Traffic flow resembles dry rice being poured through a funnel, she said. If all the grains are dumped at once, they get stuck. If poured gradually, more grains can pass through the funnel.

Drivers on I-5 will travel beneath a dozen sets of speed signs. The signs can also mark lane closures or warn of traffic jams ahead.

The $25 million installation is one of several programs meant to relieve crowding, before the eventual removal or reconstruction of the nearby Alaskan Way Viaduct. Hunter mentioned the 2009 date at a joint meeting of Seattle City Council and Metropolitan King County Councilmembers Monday, after local and state managers agreed recently to start design work.

Similar signs might be installed on the existing Highway 520 floating bridge, along with electronic toll-collection devices, if state lawmakers go ahead with a proposal to impose tolls there next year.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com

This article, originally published April 8, 2008, was corrected on April 8. The initial version of this story mistakenly reported that drivers on I-5 will travel beneath four sets of speed signs. They will travel beneath a dozen sets of speed signs.