A traveler’s best friend: New I-90/SR 18 diverging diamond interchange opens Friday, July 25

SNOQUALMIE – No longer a diamond in the rough, the newest jewel of the state highway system is ready to shine. 

The Washington State Department of Transportation opened the second diverging diamond interchange in the state early Friday, July 25, 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 opens 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.

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.

Rounding the bases on the diamond

People driving through the new I-90/SR 18 interchange should pay close attention to the lines and signs designed to safely guide travelers to their destination. The SR 18 portion of the interchange and the I-90 on- and off-ramps include:

• Freshly painted lane lines, dashed lines, directional arrows and stop bars
• Overhead signs indicating the correct lanes to keep travelers moving toward their destination
• Synchronized traffic signals to keep traffic flowing
• Salmon-colored concrete islands directing vehicles though the crossovers at each end of the diverging diamond and separating them from oncoming traffic

WSDOT opened the state’s first diverging diamond interchange in 2020 at the I-5/SR 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. 

Still polishing the diamond

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. WSDOT is finalizing dates for that work and will release details when they are available. 

A perfect marriage for better traffic flow

In addition to building the new interchange, the project is widening more than 2 miles 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.