SR 18 is one lane in each direction through this area, with truck climbing lanes on each uphill side of Tiger Mountain for the 27,000 vehicles that use the highway each day. A serious collision in the single-lane sections can require one or both directions the highway to close until it is cleared. Although other segments of the highway between Auburn and Maple Valley have been improved, this segment continues to experience congestion and serious collisions.
Move Ahead Washington was approved by state legislators in 2022 to invest $16.9 billion in major transportation projects over the next 16 years. As part of the approval, construction funding is being provided to improve SR 18 between Issaquah-Hobart Road and Deep Creek after a separate project widens the highway from Deep Creek to I-90. The preferred alternative will construct:
- Two lanes in each direction between Issaquah-Hobart Road and Deep Creek.
- Right-in, right-out access at the Tiger Mountain summit entrance.
- Roundabouts at the Issaquah-Hobart Road interchange.
- A median barrier the length of the corridor.
- Seventeen bridges and more than 60 retaining walls.
- Eleven fish passage improvements and multiple wildlife crossings.
- Stormwater improvements.
To prepare for construction, we will continue advancing the conceptual design and conduct a National Environmental Policy Act review for the more than 5-mile corridor to consider potential affects to natural and social resources in the area. NEPA and the Endangered Species Act consultation began in 2022 and will be complete before construction begins.
Between 2020 and 2021 we engaged regional leaders to support the process and help refine the design. We developed a Technical Advisory Committee in 2020 with representatives from state, tribal, and local governments, environmental and resource agencies and non-governmental organizations. The TAC supported the process by identifying key design features of the preferred alternative. An Executive Advisory Group of local, state, tribal and non-governmental leaders was also convened later that year to advise WSDOT and share project information with their communities.
Collisions and injuries (2008-2021)
- There have been more than 400 collisions on SR 18, involving 740 vehicles.
- There have been six fatalities and more than 170 people injured.
A more detailed study of crashes between 2014 and 2019 showed that 48% of crashes took place near the Tiger Mountain summit trailhead. Widening the highway, completing the installation of a center barrier and redesigning access to the Tiger Mountain parking area at the summit will reduce the risk and frequency of collisions.
Addressing traffic congestion
Existing congestion will continue to worsen without corridor improvements. Eastbound morning traffic generally backs up between the Tiger Mountain summit and the Issaquah-Hobart Road interchange. In the westbound direction, traffic backs up during both the morning and afternoon commutes with congestion stretching between Deep Creek and I-90.
Initial traffic modeling using design year 2045 to project traffic with and without corridor improvements indicates the following:
Eastbound a.m. between Issaquah-Hobart Road and I-90
- No corridor improvements: congestion increases more than 100%, increasing travel times from 31 minutes to more than an hour. Backups will extend beyond Issaquah-Hobart Road.
- With corridor improvements: Travel times will decrease more than 70% from 62 minutes to just more than 11 minutes.
Eastbound p.m. between Issaquah-Hobart Road and I-90
- Only marginal changes from existing traffic since peak use is not eastbound in the afternoons.
Westbound a.m. between I-90 and Issaquah-Hobart Road
- No corridor improvements: SR 18 travel times decrease approximately 4% due to widening of SR 18 between I-90 and Deep Creek as part of the I-90/SR 18 interchange improvement project.
- With corridor improvements: Peak morning use is not in the westbound direction, but morning travel times will decrease by approximately 10%.
Westbound p.m. between I-90 and Issaquah-Hobart Road
- No corridor improvements: Travel times will increase by more than 90%, from about 12 minutes to 24 minutes. Backups will extend from west of the Tiger Mountain summit to the I-90 interchange.
- With corridor improvements: Travel times are reduced from nearly 24 minutes to about 10 minutes.
I-90/SR 18 interchange improvements and widening
As part of a separately funded project extending from Deep Creek to I-90, WSDOT is also improving the eastern end of the highway to provide additional capacity, improve safety and reduce existing congestion. Construction for that project is expected to begin in 2022 and be completed in 2025.
This project has received $665.9 million for design and construction:
- $26 million was awarded during the 2019-2021 biennium for conceptual design and right-of-way acquisition.
- Move Ahead Washington is awarding the remaining $639.9 million to complete design and construction.
Learn more about the separate I-90/SR 18 interchange improvement project that is widening the highway east of the SR 18 widening project.