Drivers on the Eastside should prepare for a rare full closure of Interstate 405 this weekend, so contractors can demolish the Main Street overpass in downtown Bellevue.

It will be replaced by a longer bridge, so the Washington State Department of Transportation can add lanes to the freeway below.

Work could close Main Street for up to five months, between 112th and 116th avenues Northeast.

The full freeway closure, for the demolition, is to start at 10 p.m. Friday, until early Monday, June 20, in both directions of I-405 between Northeast Fourth Street and Southeast Eighth Street — almost the entire downtown core of Bellevue.

“This closure could cause significant delays throughout the weekend,” Bellevue transportation officials warn.

WSDOT’s southbound detour begins at the Northeast Fourth Street exit, continuing primarily on 112th Avenue Northeast past downtown Bellevue, until the Southeast Eighth Street interchange. Northbound drivers can exit at Southeast Eighth onto 116th Avenue Southeast and follow that arterial past downtown, to reenter the freeway at Northeast Fourth.

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Spillover traffic might clog Interstate 5 in Seattle, for people trying that path between Tukwila and Lynnwood instead of I-405.

However, the state will skip its usual weekend expansion joint repairs in Sodo this weekend because of the Juneteenth holiday, so travelers will at least avoid a double freeway closure.

WSDOT will soon construct one I-405 express lane each direction, a $705 million expansion serving toll payers, carpools, and freeway buses between Renton and Bellevue. The I-405 express lanes, along with a few general-traffic exit lanes at clogged interchanges, are scheduled to be done by late 2024, a WSDOT project description says. Sound Transit is scheduled to inaugurate its Stride bus-rapid transit line there by 2026, reaching freeway stations from Burien to Renton and Bellevue.

The WSDOT-led I-405 program will include a 2.5-mile segment of the 42-mile Eastrail where several projects are underway. By 2024, a paved walk-bike trail is scheduled to open on the wood-supported Wilburton train trestle, overlooking the freeway in Bellevue.