Washington state highway closing to traffic
A project to improve traffic flow in Washington state includes work on state Route 18 over Tiger Mountain, scheduled to begin this week.
The Washington State Department of Transportation is expanding more than 2 miles of Route 18 to two lanes of traffic in each direction as part of the Interstate 90/state Route 18 Interchange Improvements project.
In July 2025, a diverging diamond interchange was opened to complement the lanes of traffic to be added on state Route 18.
The new lanes will eliminate a pinch point south of I-90 and reduce congestion and backups at this interchange once the final configuration of Route 18 is completed, WSDOT said.
A full closure to traffic to complete work on Route 18 will begin on Thursday, Oct. 16.
During this closure, scheduled through 5 a.m. Pacific on Monday, Oct. 20, no traffic between Issaquah-Hobart Road and I-90 will be diverted. Only local traffic to Southeast 104th Street south of I-90 will be permitted during the closure. No vehicles will be allowed over Tiger Mountain summit.
A detour utilizing state Route 169, Interstate 405, state Route 900 and I-90, will be in place. Drivers are advised to consider alternate routes.
Freight traffic should use I-405 and I-90, avoiding Issaquah-Hobart Road and Issaquah city streets, which are not suitable for trucks, WSDOT said.
Paving, striping, barrier and guardrail installation and a configuration shift will be completed during the closure.
This Route 18 widening work will connect with the truck climbing lane south of Deep Creek, providing traffic approximately 4 miles to merge before highway lanes are reduced.
Remaining work on this project is scheduled to be completed in 2026 as weather permits. LL