The Washington State Department of Transportation (WSDOT) opened the second diverging diamond interchange in the state on July 25, 2025, when it switched to a new traffic pattern designed to improve flow and safety at the Interstate 90/State Route 18 interchange near Snoqualmie.
The new interchange opened after an eight-day, around-the-clock closure of SR 18 under the I-90 bridges to build the northern end and switch traffic into the new pattern.
Aecon is the project contractor.
A diverging diamond is different than traditional interchanges in several ways. Drivers will notice that northbound and southbound traffic on SR 18 crosses to the left side of the highway to go under the I-90 bridges before crossing back over.
Vehicles no longer have to turn left in front of oncoming traffic, allowing for fewer traffic-signal phases. Along with the SR 18 widening work expected to finish later this fall, the new configuration will help improve traffic flow through the interchange.
The diverging traffic pattern also improves safety by reducing the number of locations in the interchange where vehicles could potentially collide with each other or a pedestrian.
WSDOT opened the state's first diverging diamond interchange in 2020 at the Interstate 5/State Route 510 (Marvin Way) interchange in Lacey. The first diverging diamond in the nation opened in 2009 in Springfield, Mo., and there are now more than 200 throughout the country.
Although opening the diverging diamond interchange to traffic is a milestone for the I-90/SR 18 Interchange Improvements project, more work remains to complete construction. Crews need to add a top layer of asphalt throughout the interchange and ramps, then come back a few weeks later to paint permanent lane lines.
In addition to building the new interchange, the project is widening more than 2 mi. of SR 18 between I-90 and Deep Creek to two lanes in each direction. The wider roadway, which is expected to be finished later this fall, will work together with the diverging diamond interchange to improve traffic flow through this vital corridor for travelers and freight between southern and eastern King County.
Widening SR 18 included building new four-lane bridges over Lake Creek and Deep Creek and a new two-lane bridge over Raging River next to the existing bridge. Traffic was first shifted onto the new Lake Creek bridge in November 2023, while the Raging River and Deep Creek bridges opened to vehicles in late May 2025.
The new Lake Creek and Deep Creek bridges removed barriers to fish passage and opened wildlife corridors for elk, deer, coyotes and other species to safely pass beneath SR 18. In all, the project will remove six fish barriers and create an open-air stream under the I-90 bridges at the diverging diamond interchange.
The $188 million project was paid for with $183 million from existing gas tax funds and $5 million from the Connecting Washington funding package.
Construction began on the project in the fall of 2022, and road work is expected to finish by late 2025.
