State Route 18 will be closed in both directions near Interstate 90 for nearly six days beginning Thursday night, May 15, as construction crews build the southern end of a new diverging diamond interchange, according to the Washington State Department of Transportation.
The closure, which begins at 9 p.m. Thursday and lasts until 5 a.m. Wednesday, May 21, is part of the I-90/SR 18 Interchange Improvements project and will allow contractor crews to complete major drainage, paving and road-building tasks that would otherwise require 10 consecutive nights of ramp and intersection closures.
“This extended closure will help us finish the interchange work faster and more safely,” WSDOT said in a release. “It also minimizes the number of traffic shifts and reduces the risk of project delays.”
No traffic will be allowed on SR 18 beneath the I-90 bridges in either direction during the closure. However, I-90 traffic eastbound and westbound will not be impacted directly.
Drivers should note the following restrictions:
- The eastbound I-90 off-ramp will only allow right turns onto westbound SR 18. No left turns onto eastbound SR 18/Snoqualmie Parkway.
- The westbound I-90 off-ramp will only allow right turns onto Snoqualmie Parkway or back onto westbound I-90. No left turns onto westbound SR 18.
- Eastbound SR 18 drivers will be detoured to eastbound I-90.
- Southbound Snoqualmie Parkway drivers must use the westbound I-90 on-ramp and follow detour signage.
WSDOT scheduled the work between Mother’s Day and Memorial Day weekends to reduce disruption during busy travel periods. The work is weather-dependent and could be postponed in case of rain.
During the closure, crews will:
- Install drainage and electrical crossings
- Build new concrete islands for the interchange
- Pave the realigned roadway
- Construct a left-turn pocket for westbound SR 18 onto Southeast 104th Street
- Create a signalized U-turn lane south of I-90, enabling eastbound SR 18 traffic to access westbound lanes toward Auburn
WSDOT said both the left-turn pocket and U-turn lane will open when SR 18 reopens, though additional construction under the I-90 bridges and on the interchange’s northern end will continue through the summer.
Drivers should use detour loops via the Preston-Fall City (Exit 22) and Southeast North Bend Way (Exit 27) interchanges. Freight drivers on westbound I-90 can use Exit 22 or continue to southbound I-405.
Issaquah-Hobart Road and local Issaquah streets are not safe or legal options for semi-trucks and should be avoided.
All travelers should expect delays and consider alternate routes. WSDOT encourages checking the agency’s app, travel map, or social media for real-time updates.
When SR 18 reopens on May 21, the southern half of the diverging diamond interchange will be mostly complete. The final traffic shift into the full diverging diamond configuration is scheduled for later this summer.
The interchange work is part of a broader project that includes:
- Widening more than two miles of SR 18 south of I-90 to two lanes in each direction
- Building new four-lane bridges over Lake and Deep creeks to allow for fish and wildlife passage
- Constructing a new two-lane bridge over Raging River alongside the existing SR 18 bridge
The entire SR 18 widening effort is expected to be finished by the end of the year.
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