The Washington State Department of Transportation (WSDOT) is planning a full closure of SR 18 between I-90 and Issaquah-Hobart Road from Thursday night, Oct. 16 through early Monday morning, Oct. 20, to shift the highway into its final, widened configuration between I-90 and Deep Creek.
When SR 18 reopens early Monday morning, Oct. 20, the highway will be two lanes in each direction for more than 2 miles south of I-90. The opening of these new SR 18 lanes is anticipated to improve traffic flow in the new Diverging Diamond Interchange that opened in July 2025.
Please note: if the weekend forecast shows too much rain, the closure could be postponed and SR 18 widening delayed.
What to expect
From 9 pm Thursday, Oct. 16, to 5 am Monday, Oct. 20, no eastbound or westbound SR 18 traffic will be allowed between Issaquah-Hobart Road and I-90. Only local access traffic to SE 104th Street will be permitted south of I-90 during the closure, and no vehicles will be allowed over Tiger Mountain summit.
A signed detour will use SR 169, I-405, SR 900 and I-90, and people should consider other routes. Freight traffic should use I-405 and I-90 throughout the four-day closure and avoid Issaquah-Hobart Road and Issaquah city streets, which are not suitable for semi-trucks.
During the closure, crews will:
- Pave transitions.
- Stripe lanes in the new configuration with two lanes in each direction.
- Install required barrier and guardrail.
- Shift SR 18, including the intersection with Southeast 104th Street, into its final configuration.
Improved traffic flow
The work is part of the I-90/SR 18 Interchange Improvements project, which also opened a diverging diamond interchange in July 2025. The new interchange was designed to work with the additional lanes of SR 18 to improve traffic flow and safety through the corridor.
The new lanes will eliminate a pinch point a few hundred feet south of I-90. WSDOT expects congestion and backups at the I-90/SR 18 interchange to reduce once SR 18 is placed in its final configuration.
Construction is expected to finish in 2026, when a top layer of asphalt will be placed throughout the diverging diamond interchange and the new lanes of SR 18.
Real-time travel information is available via the WSDOT mobile app, the WSDOT travel map or by signing up for WSDOT's email updates.