Summer traffic jams at the Montlake drawbridge will spread this weekend, when state contractors close the entire Highway 520/Montlake Boulevard interchange for additional roadwork.

The big shutdown, from 10 p.m. Friday until 5 a.m. Monday, allows crews to temporarily shift all north-south lanes of the boulevard onto a new concrete lid that the Washington State Department of Transportation recently built for its $4.65 billion Highway 520 megaproject. Contractors are also removing toxic soils beneath a former gas station.

Once traffic is temporarily cruising the lid, state contractor Graham Construction can build a new interchange, nine lanes wide over 520, expected to open in late 2022.

The weekend shutdown blocks Montlake Boulevard East from East Roanoke Street to the drawbridge, closes three Highway 520 ramps, and closes East Lake Washington Boulevard.

Pedestrians and bicyclists can still travel through a narrow path being maintained this weekend along Montlake Boulevard. Drivers in the immediate neighborhood can wait for a pilot car, to caravan at 10 mph through that same path, said Steve Peer, state Highway 520 spokesperson. Travelers on foot and bikes could also use side streets and the state trail to pass under the highway, about one-quarter mile to the east, he said.

Sound Transit’s light rail tunnel is operating normally, as a quick option to reach the University of Washington from points south. The Seattle Department of Transportation has striped a westbound transit lane on Northeast Campus Parkway to help reduce delays for Montlake buses detouring via University Bridge.

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All areas are closed this weekend. (Washington State Department of Transportation)

Interchange work is in addition to the unrelated steel deck-replacement project on the drawspan over Montlake Cut, from Aug. 9 until early Sept. 3. The deck remains on pace to reopen on time.

Why schedule the interchange job this month, when traffic is already fouled by urgent drawbridge repairs?

“It was more motivation to do it now, because we didn’t want to interrupt the travelers twice,” Peer said. The drawbridge work eliminated most of the normal 60,000 Montlake Boulevard trips, so 520’s ramp closures affect fewer drivers than if WSDOT shifted the interchange lanes later.

In other detours, Interstate 90 closes in both directions from 11:59 p.m. Friday until 6 a.m. Saturday, for overnight inspections, equipment deliveries, and maintenance between I-5 and Island Crest Way.

Eastbound Highway 18 will be reduced to one lane at Auburn-Black Diamond Road from 2 a.m. until noon Saturday, for maintenance work.