A quarter-million travelers will need to detour or cancel their trips into Seattle this weekend, as construction closures block Highway 520, Highway 99 and Sound Transit light rail downtown.

Highway 520 westbound closes from 11 p.m. Friday until 5 a.m. Monday, from 92nd Avenue Northeast on the Eastside to Montlake Boulevard, so crews can install a concrete median barrier to divide the three-lane westbound deck built in 2017.

Afterward, the carpool lane westbound will expire 1 1/2 miles farther out at mid-Lake Washington, so its 7,500 drivers a day will merge sooner with 37,500 vehicles in the general lanes.

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Then on Oct. 28, both directions of traffic will squeeze into that new westbound span passing Foster Island, while the remaining 1963 highway span over Union Bay that serves eastbound traffic is replaced by a seismically stronger bridge.

The speed limit through this curved detour will be 40 mph during the three to four years of bridge construction.

Contractors will also build a new Montlake-area landscaped lid, vehicle ramps and wider intersections.

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The current layout has supplied six lanes, all the way to Montlake Boulevard.

The work is part of a decades-long, $4.6 billion project to rebuild Highway 520 from the Eastside across Lake Washington to Interstate 5.

The floating bridge’s walk-bike trail also closes this weekend, so workers can replace steel expansion joint plates with thinner plates that are easier on bicycle tires.

King County Metro bus routes 255 and 271, and Sound Transit 545, from the Eastside will detour using Interstate 405 and the Interstate 90 floating bridge. Route 271 will continue from I-90 to reach its regular University District stops, and the other two routes will serve all downtown Seattle destinations.

Highway 99 northbound through downtown Seattle closed starting at 11 p.m. Friday, while crews repaired a failed road surface at the Sodo entrance to the new $2.1 billion tunnel. The highway reopened early around midday Sunday, according to the Washington State Department of Transportation (WSDOT).

Seattle Tunnel Partners is paying for the fix under warranty, said tunnel spokeswoman Laura Newborn. When contractors poured the concrete, it didn’t flow through closely spaced rods of steel rebar, leaving a gap where pavement didn’t bind, Newborn said.

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WSDOT decided to close all northbound lanes at Spokane Street, instead of letting drivers continue to the First Avenue South/Downtown exit this weekend.

The ramp shutdown enables paving-company equipment to use portions of the interchange, said state traffic spokeswoman Lisa Van Cise. WSDOT staff also believe that diverting all highway traffic through the First Avenue South/Downtown exit would cause traffic jams all the way back to Spokane Street, she said.

Therefore, drivers arriving from SeaTac and Burien must exit Highway 99 into the tangle of East Marginal Way truck routes, or bail out farther upstream at First Avenue South, immediately after crossing the First Avenue South river bridge.

Drivers from the West Seattle Bridge will miss their loop ramp to Highway 99, and detour down to First Avenue South, or Fourth Avenue South, as occurred when the Alaskan Way Viaduct was closed in January.

Sound Transit light rail will miss downtown Seattle from 11 p.m. Friday until early Monday, so workers can build a temporary passenger-boarding platform between the rails at Pioneer Square Station, for use this winter.  Westlake, University Street, Pioneer Square, International District and Stadium stations will close this weekend.

Trains will run every 10 minutes between University of Washington and Capitol Hill stations, and separately between Angle Lake and Sodo stations. Shuttle buses will run between Sodo and Capitol Hill, stopping at all five closed stations.

This work prepares for a 10-week disruption in January when tracks for the 2023 Bellevue corridor are joined to Seattle tracks next to International District/Chinatown Station. Dual tracks there will be reduced to a single track, so trains will reverse at Pioneer Square.  To continue through downtown those 10 weeks, an estimated 30,000 of the system’s 80,000 daily riders must switch trains on that center platform within Pioneer Square Station.

All three of this weekend’s closures were planned to avoid the Oct. 19-20 weekend, when the Sounders FC, Seahawks and Washington Huskies football team play home games, along with a concert by The Who at T-Mobile Park.