Drivers should prepare for a full closure of State Route 18 over Tiger Mountain beginning Thursday night, according to the Washington State Department of Transportation.
From 9 p.m. Thursday, Oct. 16, through 5 a.m. Monday, Oct. 20, all lanes of SR 18 will be closed between Issaquah-Hobart Road and Interstate 90 while contractor crews expand the highway to two lanes in each direction from I-90 to Deep Creek.
Only local traffic to Southeast 104th Street will be allowed south of I-90 during the closure.
The work marks a key milestone in the I-90/SR 18 Interchange Improvements project, which opened the state’s second diverging diamond interchange last summer.
That interchange was designed to improve traffic flow and safety but hasn’t yet reached full potential because of a “pinch point” south of I-90 where multiple traffic streams merge into a single lane.
Once this section is widened, traffic is expected to move more smoothly through the corridor.
During the closure, drivers should use a signed detour route that includes SR 169, I-405, SR 900 and I-90.
STOP PINCHING
— WSDOT Traffic (@wsdot_traffic) October 15, 2025
This pinch point along WB SR 18 just south of I-90 will soon be gone, improving traffic flow in this busy corridor.
To remove it and widen SR 18, we will close EB & WB SR 18 over Tiger Mtn. from 9p Thurs (10/16) to 5a Mon (10/20).
More: https://t.co/tPd8Nvcv5d https://t.co/spUTSebC9u pic.twitter.com/OrEovd8kaS
WSDOT is asking freight haulers to use I-405 and I-90 and to avoid Issaquah-Hobart Road and Issaquah city streets, which are not designed for heavy truck traffic.
Crews will pave transitions, stripe lanes in the new configuration, install barriers and guardrails, and shift SR 18 and its intersection with Southeast 104th Street into their final design.
WSDOT said the work depends on dry weather, and the closure could be postponed if the weekend forecast calls for heavy rain.
The new configuration will remove the bottleneck where SR 18 narrows shortly after the interchange.
The widened highway will continue westbound to Tiger Mountain summit, connecting with the existing truck climbing lane south of Deep Creek.
That will give westbound drivers about four miles to merge before SR 18 reduces to a single lane.
WSDOT expects the change to reduce congestion and backups that currently extend from the SR 18 interchange up both I-90 off-ramps and onto mainline I-90 during busy times.
Although traffic will shift into its permanent configuration after this closure, several parts of the project will continue into 2026.
Crews still need to complete final paving through the interchange and along both directions of SR 18, permanent lane striping, and landscaping work.
These tasks will be finished once the weather improves.
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