Headed to the West Side? Washington Route 18 interchange work to cause days of detours at I-90 in King County

SEATTLE – One of Washington state’s main driving routes will be blocked for five days during reconstruction where Interstate 90 exits into state Route 18.

All SR 18 lanes below the I-90 bridges at Exit 25/Snoqualmie Parkway in Issaquah will close from 9 p.m. Thursday until 5 a.m. Wednesday with detours that will likely cause lengthy delays on the already-clogged route.

Contractors for the Washington State Department of Transportation are building a new junction that WSDOT hopes will reduce congestion, along with rear-end crash risks when traffic pileups spill into westbound I-90’s right lanes. An average of 12,500 vehicles per day, including cargo trucks headed to the Port of Tacoma, take Exit 25 from westbound I-90. Traffic is heavier on warm summer weekends.

When finished, it will be a “diverging diamond” interchange, similar to where Marvin Road crosses over Interstate 5 in Lacey. Drivers will navigate a slight bend to find themselves on the left half of a boulevard, reducing left-turn risks and backups at traffic signals. At least 230 of these types of interchanges have been built or are under construction nationally, WSDOT says.

State officials say the painful five-day blockage under I-90 will prevent a more piecemeal approach from roadwork dragging on for months of weekend and night closures.

Drivers going from Auburn to Eastern Washington can still take Highway 18 to reach eastbound I-90 and Snoqualmie Pass. But they can’t bend west toward Bellevue there, nor can Snoqualmie and Fall City drivers go from Snoqualmie Parkway directly to eastbound I-90.