I-90 – SR 18 Interchange Improvements

What is changing

• Creating a diverging diamond interchange.
• Widening two miles of SR 18, providing two lanes in each direction.
• A traffic signal-controlled U-turn just west of the interchange.
• A new 2-lane bridge over Raging River next to the existing bridge.
• Two new 4-lane bridges, one at Deep Creek and one at Lake Creek.
• Replacing fish passage barriers with relocated, open channel streams.
• Improving habitat connectivity for wildlife, including deer and elk.

What we've done so far

The first stage of construction began in April 2023. Contractor crews cleared trees in the project area to begin work on the new interchange and highway widening. 

Since then, we have:

• Switched traffic at the I-90/SR 18 interchange into the diverging diamond pattern.
• Built a new four-lane SR 18 bridge over Lake Creek.
• Built a new two-lane SR 18 bridge over Raging River next to the existing bridge.
• Built a new four-lane SR 18 lane bridge over Deep Creek.
• Built two new lanes of SR 18 between Deep Creek and Lake Creek, allowing us to shift both directions of traffic to the new asphalt and repair the existing lanes.
• Widened SR 18 to two lanes in each direction between Deep Creek and I-90.
• Constructed a new open-air channel next to SR 18 to allow runoff water and fish to pass beneath the I-90 bridges.
• Installed two culverts that will run beneath the westbound I-90 off-ramp and eastbound I-90 on-ramp.
• Removed the old weigh station near the westbound I-90 on-ramp.

A variety of construction activities are taking place throughout the project area. Some of this work includes:

• Adding a top layer of asphalt throughout the project, including on SR 18 from I-90 to Deep Creek, through the diverging diamond and on the I-90 on- and off-ramps at SR 18.
• Finish installing guardrail and traffic loops.
• Complete drainage work.
• Ongoing maintenance and roadway repairs as needed.

Diverging diamond interchange

A diverging diamond interchange is a proven way to keep traffic flowing and improve safety by reducing the potential for collisions. The I-90/ SR 18 diverging diamond interchange reduces conflict points from 26 to 14, which in turn, reduces the opportunity for collisions by 50%.

In a traditional interchange, three phases of traffic signals are required to move traffic through the intersection, including separate phase for left turns. In a diverging diamond interchange, through-traffic and left turns happen at the same time, eliminating the need for a separate left-turn phase. 

The first diverging diamond interchange in the United States was built in 2009 in Springfield, Mo. More than 200 of these interchanges are now open nationwide, and more are being built. Washington state's first DDI was the I-5/Marvin Road interchange in Lacey.

Removing fish barriers & enhancing wildlife 

As part of this project, we removed six fish passage barriers in the area. We also installed two new bridges that will allow fish to pass under SR 18 at Deep Creek and Lake Creek. These new stream crossings restored access to 13 miles of stream habitat that is essential for populations of salmon, steelhead and other aquatic species. The new bridges built for highway widening also allowed wildlife to pass safely beneath roadway instead of over it.

In the Deep Creek watershed, this project opened access to 11.2 miles of stream habitat including about .75 acres of spawning area and 2.34 acres of rearing habitat. These fish are important resource for Tribes through their treaty rights with Washington and also as forage for endangered Orca in Puget Sound.

Low-cost improvements

WSDOT has kept a close eye on the I-90/SR 18 interchange for several years. When possible, low-cost improvements have been implemented to enhance safety and keep vehicles moving as efficiently as possible as more people move to Snoqualmie Ridge.

• Extended the "exit only" striping on the eastbound I-90 approach to the SR 18 exit.
• Installed two new active warning signs on eastbound I-90 to notify travelers of stopped traffic ahead. The first sign is approximately two miles before the I-90/SR 18 interchange, and the second sign is about a mile before the interchange.
• Widened the eastbound I-90 off-ramp to SR 18.
• Changed the right lanes of eastbound and westbound I-90 to exit-only lanes.
• Partnered with the city of Snoqualmie to change the lane configuration on southbound Snoqualmie Parkway - making the southbound right lane a right-turn-only lane to the I-90 westbound on-ramp.
• Adjusted traffic signal timing at the two signalized intersections to operate the interchange as efficiently as possible.
• Installed “Thru traffic keep left” signs on both directions of I-90 prior to the SR 18 interchange to guide through-traffic to stay in the left two lanes and keep the right lanes clear for drivers exiting to SR 18 or Snoqualmie Parkway.
• Installed “No U-turn” signs on SR 18 at the interchange to prohibit U-turns that have caused congestion for the off-ramp traffic.

Community outreach

WSDOT is conducting a thorough and extensive community outreach effort throughout this project.

• Watch for popup outreach events as we get closer to opening the diverging diamond interchange.
• WSDOT hosted booths at the 2024 Snoqualmie Days (Aug. 17) and Issaquah Salmon Days (Oct. 5-6) events to share project updates. We also hosted a booth at the 2023 Issaquah Salmon Days event.
• In November 2022, WSDOT and design-builder Aecon hosted an in-person open house at Snoqualmie Elementary School with a chance to talk with project managers and drive the diverging diamond interchange simulator.
• In summer 2022, the project office and contractor teams hosted booths at three farmers markets in North Bend, Maple Valley and Snoqualmie to boost awareness about the project to local residents.
• In summer and fall 2018, the project team hosted booths at fairs and festivals and conducted stakeholder interviews.
• WSDOT formed a Stakeholder Advisory Group (SAG) for this project. A SAG is a group of individuals representing local government, businesses, transit, and other interested agencies and organizations. SAG members provided critical guidance, advice and recommendations to WSDOT to help select a preferred design.
• WSDOT held six community briefings in March and April 2019 to update community groups on the project and get their feedback on the design proposals.
• In April 2019, WSDOT hosted two in-person open houses and an online open house to present designs for the new I-90/SR18 interchange and widening SR 18 to Deep Creek.

Outreach continues through a series of advisory meetings, community briefings and future open houses.

SE 104th Street improvements

As part of the I-90/SR 18 interchange improvement project, WSDOT added active warning signs with flashing yellow beacons south of the Southeast 104th Street intersection to alert drivers on eastbound SR 18 of vehicles entering/exiting the highway at the intersection. 

This project is being delivered through a method called design-build.