Drivers should prepare for a complete closure of State Route 18 over Tiger Mountain. The Washington State Department of Transportation (WSDOT) announced this closure will begin Thursday night. This work is part of a larger project to expand the highway.
The closure will affect all lanes of SR 18 between Issaquah-Hobart Road and Interstate 90. It starts at 9 p.m. on Thursday, October 16, and will last until 5 a.m. on Monday, October 20. Contractor crews will use this time to expand the highway to two lanes in each direction. This section runs from I-90 to Deep Creek.
Key Takeaways
- State Route 18 over Tiger Mountain will close fully from October 16-20.
- The closure is for widening the highway to two lanes in each direction.
- A signed detour route is in place, using SR 169, I-405, SR 900, and I-90.
- Freight haulers should avoid Issaquah-Hobart Road and Issaquah city streets.
- The work depends on dry weather conditions.
Details of the Closure and Detour Route
During the closure, only local traffic heading to Southeast 104th Street will be allowed south of I-90. All other drivers will need to use an alternative route. WSDOT has provided a clear detour plan.
The signed detour route includes several state routes and interstates. Drivers should use SR 169, then I-405, followed by SR 900, and finally I-90. This route will help commuters navigate around the closed section of SR 18.
Important Fact
The I-90/SR 18 Interchange Improvements project opened the state’s second diverging diamond interchange last summer. This design aims to improve traffic flow and safety at the junction.
WSDOT has specific instructions for freight haulers. They are asked to use I-405 and I-90. It is important for large trucks to avoid Issaquah-Hobart Road and Issaquah city streets. These local roads are not designed to handle heavy truck traffic, and using them could cause further delays and damage.
The work schedule is weather-dependent. Crews need dry conditions to complete the necessary tasks. If heavy rain is forecast for the weekend, the closure could be postponed. Drivers should check WSDOT updates before traveling.
Project Milestones and Future Benefits
This work marks a significant step in the I-90/SR 18 Interchange Improvements project. The project previously introduced a diverging diamond interchange. This interchange was designed to enhance traffic flow and safety. However, a 'pinch point' south of I-90 has limited its full potential.
This 'pinch point' occurs where multiple traffic streams merge into a single lane. Once this section of the highway is widened, traffic is expected to move more smoothly. This will improve overall efficiency through the corridor, especially during peak travel times.
Project Background
The broader I-90/SR 18 Interchange Improvements project aims to modernize a key transportation corridor. It addresses long-standing congestion issues and enhances safety for thousands of daily commuters and freight operators in the region.
The new configuration will eliminate the current bottleneck. This bottleneck forms where SR 18 narrows shortly after the interchange. The widened highway will extend westbound to the Tiger Mountain summit. It will connect with the existing truck climbing lane south of Deep Creek.
This expansion will give westbound drivers about four miles to merge. This merging distance is crucial before SR 18 reduces back to a single lane. WSDOT anticipates this change will significantly reduce congestion and traffic backups.
"Once this section is widened, traffic is expected to move more smoothly through the corridor," a WSDOT spokesperson stated, highlighting the anticipated benefits of the project.
Work During the Closure Period
During the four-day closure, crews will perform several critical tasks. These tasks are essential for the new highway configuration. One primary activity is paving transitions. This involves laying down asphalt to connect the new lanes with existing road surfaces.
Another key task is striping lanes in the new configuration. This ensures clear markings for drivers. Crews will also install new barriers and guardrails. These safety features are vital for protecting motorists along the widened highway.
Finally, SR 18 and its intersection with Southeast 104th Street will be shifted into their final design. This involves adjusting the alignment of the road to match the expanded capacity. All these operations are highly dependent on favorable weather.
Long-Term Project Outlook
While traffic will shift into its permanent configuration after this closure, the overall project will continue into 2026. This means several parts of the project are still pending. The upcoming work includes final paving through the interchange and along both directions of SR 18.
Crews also need to complete permanent lane striping. This final layer of striping will be more durable than temporary markings. Landscaping work is another component that will be finished at a later stage. These remaining tasks will be completed once the weather conditions improve, typically in warmer months.
WSDOT expects the changes to reduce backups that currently extend from the SR 18 interchange. These backups often reach both I-90 off-ramps and onto the mainline I-90 during busy periods. The project aims to provide a more reliable and less congested route for drivers.
Traffic Impact
WSDOT expects the widening to reduce backups that currently extend from the SR 18 interchange onto both I-90 off-ramps and mainline I-90 during busy times.
The long-term benefits of this project include improved safety and reduced travel times. Drivers will experience a more efficient journey through the Tiger Mountain corridor. The expansion also supports regional economic activity by facilitating smoother freight movement.
Commuters are advised to plan their routes in advance and allow extra travel time during the closure. Real-time traffic information and project updates are available through WSDOT's official channels. Staying informed will help minimize disruptions.
