State Route 18 will close for four days over Tiger Mountain for a widening project.
From 9 p.m. Thursday, Oct. 16, to 5 a.m. Monday, Oct. 20, no eastbound or westbound SR 18 traffic will be allowed between Issaquah-Hobart Road and I-90.
When SR 18 reopens, the highway will be two lanes in each direction for more than 2 miles south of I-90.
According to the Washington State Department of Transportation (WSDOT), much of the work requires dry weather, and the closure may need to be rescheduled if there’s too much rain.
Only local access traffic to Southeast 104th Street will be permitted south of I-90 during the closure, and no vehicles will be allowed over Tiger Mountain summit.
There will be a signed detour, directing traffic onto SR 169, I-405, SR 900, and I-90.
WSDOT says freight traffic should use I-405 and I-90 and avoid Issaquah-Hobart Road and Issaquah city streets, which are not suitable for semi-trucks.
During the closure, WSDOT 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.
The work is part of the I-90/SR 18 Interchange Improvements project, which also opened a diverging diamond interchange in July.
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. That’s when crews will add a top layer of asphalt throughout the diverging diamond interchange and the new lanes of SR 18.
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