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I-90 diverging diamond will put drivers on wrong side of the road

A major renovation is planned for the I-90 / Highway 18 interchange and the fix will force drivers over to the wrong side of the road.

The “diverging diamond” is intentional and it’s a new way of thinking about major freeway interchanges. It’s only been in use since 2009, with more than 80 built all over the United States.

Washington’s first diverging diamond was designed for Lacey. Now, one will be implemented at the I-90 and Highway 18 interchange.

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Imagine you’re traveling on eastbound Highway 18 and want to go west on I-90 toward Issaquah. As you approach the new overpass, you will veer left and drive over I-90 on the left side of the road. You will then take a left onto the ramp to I-90 without waiting. If you are going to Snoqualmie, you would veer to the right at the end of the overpass. Traffic coming from Snoqualmie would do the same thing in the opposite direction.

“It has far less conflict points,” said John White with the Washington State Department of Transportation. “It’s safer, much more flee-flowing. The right turns are free-flowing. The left turns are what you would call free-flowing for the most part. So it leads to a much more efficient, compact interchange.”

White says the beauty of the diverging diamond is one direction moves at a time, clearing out all the traffic while avoiding wait time for a left-turn signal.

“Far more efficient,” he said. “You have less signal cycles, you have less chance for conflict and accidents, and people turning out and cutting off people because you have free-flowing movements that aren’t conflicting with other traffic movements.”

The Oregon Department of Transportation completed its first diverging diamond project last year. ODOT spokesperson Gary Leaming says drivers adopted it right away.

“You have fewer crashes and also you can move a large amount of traffic through the diverging diamond interchange, unlike a normal diamond interchange,” Leaming said. “So they are very efficient as well as very safe.”

This type of interchange also takes up a much smaller area.

White says the state was planning a huge interchange with I-90 and Highway 18 that included massive fly-over ramps. Its footprint would have been much bigger and more expensive than this one.

The project breaks ground next month, thanks to a lot of lobbying in Olympia. It was originally planned for initial funding in 2023.

The new interchange should be finished by the time it was originally planned to begin, which is great news for people in Snoqualmie and all the truck drivers that travel through every day.

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