Thousands of Redmond and Sammamish drivers must detour the next four months, while light-rail construction blocks the busy Redmond Way onramp to westbound Highway 520.

Sound Transit contractors will raise the highway ramp, so future Link trains can roll beneath while entering and leaving downtown Redmond, according to project spokesperson Rachelle Cunningham.

The closure begins Friday at 9 p.m., she said.

Sound Transit is scheduled to finish the $1.5 billion, 3.4-mile Redmond extension by Dec. 31, 2024, which might be delayed by the recent concrete truck drivers strike. Two stations will be added — one in downtown Redmond and one in Marymoor Park. Before that, the connecting tracks between the Microsoft campus, Bellevue and Seattle’s International District/Chinatown Station should open by mid-2023.

Redmond Way is part of state Highway 202, which continues south from downtown past Sammamish and Fall City, before linking to Interstate 90. The westbound ramp to Highway 520 serves an average of 3,200 vehicles daily that will have to go elsewhere.

The official alternate route winds past downtown along Bear Creek Parkway and Northeast Leary Way, where motorists can proceed onto westbound Highway 520 as it climbs uphill. Congestion could increase around that entrance, where drivers already experienced a 37-second average delay in morning peak hours, pre-pandemic.

Ramp-metering signals at the Leary Way entrance to Highway 520 will be monitored and adjusted as needed, but no other changes are planned to those onramp lanes, said Elizabeth Guevara, spokesperson for the Washington State Department of Transportation.

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Contractors built an alternate route near the eastbound Highway 520 offramp to Redmond Way last year, but it remained open for traffic because there was enough space to build a temporary lane nearby, Guevara said.

This spring’s project will also add a segment to the East Lake Sammamish Trail, closing a gap in the regional trail network through Redmond, said Kelli Refer, spokesperson for Move Redmond, a nonprofit advocacy group for transit, cycling and walking.

“The ultimate goal of the construction is really great and moves us in a positive direction,” she said.