The Washington State Department of Transportation (WSDOT) opened the state’s second diverging diamond interchange early Friday, July 25, improving traffic flow and safety at the busy I-90/SR 18 interchange near Snoqualmie.
The new configuration opened following an eight-day, 24-hour closure of SR 18 under the I-90 bridges to complete construction of the northern end and transition vehicles into the new traffic pattern.
How the Diverging Diamond Works
Unlike traditional interchanges, the diverging diamond shifts northbound and southbound SR 18 traffic to the left side of the roadway to travel beneath I-90 before crossing back. This design eliminates the need for drivers to make left turns across oncoming traffic, reducing signal phases and potential conflict points for both vehicles and pedestrians.
The diverging diamond is part of a broader project to widen SR 18. Together, the improvements are expected to ease congestion through this critical corridor.
New Features in Place
Drivers will notice several updates designed to guide them through the new interchange safely, including:
- Fresh lane striping, dashed lines, arrows, and stop bars
- Overhead signs marking correct lanes
- Synchronized traffic signals
- Salmon-colored concrete islands at crossover points to separate traffic
Washington’s first diverging diamond opened in 2020 at the I-5/SR 510 interchange in Lacey. The concept was first introduced in the U.S. in 2009 in Springfield, Missouri, and now appears in more than 200 locations nationwide.
Additional Work Ahead
Crews will return to the site later this year to add a final asphalt layer and permanent lane striping. WSDOT will announce those dates once scheduled.
SR 18 Widening and Wildlife Improvements
The interchange upgrade is one part of a larger effort to widen more than two miles of SR 18 between I-90 and Deep Creek. Upon completion in late 2025, this stretch will feature two lanes in each direction, enhancing both traffic flow and freight movement.
The project also features new four-lane bridges over Lake Creek and Deep Creek, as well as a new two-lane bridge over the Raging River, which opened to traffic in May 2025. Traffic shifted to the new Lake Creek bridge in November 2023.
Beyond road improvements, the project removes six fish barriers and creates an open-air stream beneath I-90 to support salmon migration. The new Lake Creek and Deep Creek bridges also open wildlife corridors for elk, deer, coyotes, and other animals to pass under SR 18 safely.
Project Funding
The $188 million project is funded with $183 million from existing gas tax revenues and $5 million from the Connecting Washington package. Construction began in fall 2022, with full project completion expected by late 2025.
[Information provided by the Washington State Department of Transportation (WSDOT)]
