Drivers who are planning to navigate through the Tiger Mountain area should prepare to adjust their routes temporarily. According to a report by the Washington State Department of Transportation, State Route 18 will be getting a significant upgrade, which includes the expansion of more than 2 miles to two lanes in each direction from Interstate 90 to Deep Creek. The construction is set to take place following a complete closure of SR 18 over Tiger Mountain, scheduled for Thursday night, October 16, through Monday, October 20. The closure is set to begin at 9 p.m. October 16 and end at 5 a.m. October 20, blocking all eastbound and westbound traffic except for local access to Southeast 104th Street.
The closure is due to pave the way for several essential updates to the roadway, including paving transitions and striping lanes in the new configuration. There is also a focus on safety, as crews will be tasked to install the required barrier and guardrail and to shift SR 18, including the intersection with Southeast 104th Street, into its final configuration. All this work crucially depends on dry weather, and the closure may have to be rescheduled should the forecast show too much rain. A signed detour will use SR 169, I-405, SR 900, and I-90, and alternate routes should be considered, especially by freight traffic, which is advised to avoid Issaquah-Hobart Road and Issaquah city streets during the closure.
Once the expansion is completed, it is expected to significantly improve traffic flow and safety through the I-90/SR 18 corridor. The upgrades are a part of the I-90/SR 18 Interchange Improvements project, which has already seen the opening of the state’s second diverging diamond interchange in July 2025. The new configuration aims to eliminate a problematic pinch point just south of I-90, where traffic currently narrows down to a single lane, often causing extended congestion. WSDOT expects that backups at the interchange will reduce substantially once SR 18's new lanes are open and fully functional.
Looking forward to the project's completion, there remain several tasks to be handled, including the final paving through the I-90/SR 18 diverging diamond interchange and along both directions of SR 18 from I-90 to Deep Creek. Moreover, crews need to still lay permanent lane striping and to complete landscaping, curb, and guardrail installations. These endeavors are scheduled to be finished in 2026 when the weather is expected to improve, as stated by the Washington State Department of Transportation.
