WSDOT I-90/SR 18 Interchange Improvements

Description from WSDOT

Growth in Snoqualmie Valley has made the I-90/SR 18 interchange one of the busiest in the state, causing backups during peak times and busy travel weekends. To improve traffic flow, WSDOT will build a diverging diamond interchange and add two lanes on SR 18 between the interchange and Deep Creek.

Building the diverging diamond interchange will require three construction stages lasting six to nine months each. Traffic will shift to make room for multiple work zones and drivers can expect intermittent lane and ramp closures throughout the life of the project.

The first stage of construction begins in late April/early May 2023. Contractor crews will remove the old weigh station located near the westbound I-90 on-ramp from Snoqualmie Parkway, install new utilities and build a drainage pond.

Two full weekend closures of SR 18 between I-90 ramp intersections will occur in summer 2024 when traffic shifts into the new diverging diamond configuration. 

Widening SR 18 will require lane shifts, speed limit reductions, and closing one northbound lane north of Raging River Bridge. Additional information, including detour routes, will be available during construction.


$188 million (state funded project)

More Information

Visit the WSDOT website for detailed information about this project.

I-90_SR 18-DDI looking eastDesign rendering of the I-90/SR 18 diverging diamond interchange looking east. Courtesy, Aecon.