Speed limits will vary on I-90 Floating Bridge

Starting Tuesday, the speed limit on the Interstate 90 floating bridge will no longer stay at 60 mph.

When traffic is heavy, the speed limit might temporarily be 50 or 40 mph. Ice on the roadway could force the posted speed limit down to 30 mph.

The state Department of Transportation (DOT) has installed electronic "variable speed signs" from Seattle to Mercer Island and Bellevue. Lower speeds are intended to smooth out traffic flow, a technique used in a few other states, and in Europe. For instance, instead of accelerating westbound through the I-90 Mount Baker Tunnel — then slamming on the brakes when cars are crawling into the I-5 interchange — a westbound driver might cruise along at 40 mph, starting at Mercer Island.

The DOT's theory will be tested soon, when repair work causes slowdowns. The I-90 express lanes will close Monday through May 23 to replace broken expansion joints where the fixed and floating bridge segments meet.

From July 5 to 28, the main westbound lanes close, diverting three lanes of mainline traffic into the two freshly repaired express lanes.

Similar variable-speed signs exist on Snoqualmie Pass, where the speed limits drop if snow is accumulating.

More variable-speed projects are on the way to area highways.

Next year, overhead gantries will show variable speeds on northbound I-5 alongside Boeing Field. There will also be green arrows, and a red "X" sign, if a crash blocks a lane.

Variable speeds will be introduced on the Highway 520 bridge in the next few years, said Morgan Balough, state roadway-operations engineer.

He didn't promise a general improvement in commute times — but Balough said variable speeds should reduce rear-end crashes, which cause injuries and the occasional nightmare traffic jam.

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