An example of the diverging diamond to be used in the new S.R. 18 and I-90 interchange is illustrated in a video on Snoqualmie’s website. (Courtesy Photo)

I-90/S.R. 18 interchange fixes moved up on state schedule; construction to start in 2019

Good news for Valley drivers, the problematic interchange between I-90 and State Route 18 has had its project timeline moved up seven years.

Thanks to work from State Senator Mark Mullet and State Representatives Jay Rodne and Paul Graves, the project, which was originally slated to begin in July 2023 and be completed by 2028, is now starting this year. Snoqualmie Mayor Matt Larson said the funding will be available when the state Legislature’s next fiscal year begins in July.

The new plan begins with the Washington State Department of Transportation putting out a Request for Proposal (RFP) to find a consultant to create an improved interchange design. In August, the bidding will close and a consultant will be selected.

The design process will begin in September. Design, environmental reviews and Federal Highway Administration reviews will extend into 2019, when construction is scheduled to begin. The substantial completion of the project is projected to be in 2022.

In 2015, the project received an allocation of $150 million from the state Legislature as part of that year’s $16 billion transportation package. Access to that funding, which was set to become available in 2023, was moved up because Reps. Rodne and Graves were able to get a new source of funding to speed up the process.

Larson said that there was some left-over funding from improvement work done on S.R. 18 and access to that funding is what helped push the project forward. Being able to source funding from that leftover S.R. 18 improvement project allowed more transportation funding to be available for other projects around the state.

As for the redesign of the interchange, Larson said the city came up with an unusual design that could address all of the conflicts at the interchange. The design is called the diverging diamond, which is currently implemented in 86 locations across the country.

Larson said the diverging diamond will lower the amount of conflict points at the intersection from 26 to 14 and will address them all at the same time, meaning that the project is not broken up into separate phases. The design is also cheaper than a standard flyover ramp, which would allow some of the $150 million in funding to be used by WSDOT to fund improvements along the S.R. 18 corridor.

The city of Snoqualmie has produced a video on its official YouTube channel detailing the new schedule for the interchange as well as an illustration of how the diverging diamond design will work.

Find the video at www.youtube.com/watch?v=_ZGL31BJ2gM.