I-90 – SR 18 Interchange Improvements

Diverging diamond interchange

A diverging diamond interchange is a proven way to move more people and reduce the potential for collisions by reducing the number of conflict points between vehicles. The first diverging diamond interchange in the United States was built in 2009 in Springfield, MO. Since then, over 60 of these types of interchanges are now open to traffic throughout the United States, and another 20 are currently being built.

The first diverging diamond interchange built in Washington is at the I-5/ Marvin Road interchange in Lacey. In a traditional diamond interchange, three phases are required to move traffic through the interchange, including a separate phase for left-hand 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. 

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.

• 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 104^th Street improvements

As part of the I-90/SR 18 interchange improvement project, WSDOT will also make changes to the intersection at Southeast 104th Street. The proposed configuration will improve safety for drivers entering and exiting the highway at this intersection and benefit traffic flow through the new interchange. WSDOT has proposed a two-way stop, a protected left-hand turn lane barrier, a U-turn option and a new flashing traffic signal.

This project will be delivered through a method called design-build. This type of contract authorizes the awarded firm to decide if WSDOT’s proposed traffic revision is the best solution or to design a different, more cost-effective option. This means the new intersection could look different from the one WSDOT has recommended.