Construction starting on Washington’s first diverging diamond interchange

Mar 26, 2019, 10:43 AM | Updated: 3:10 pm

Construction on the state’s first diverging diamond interchange is about to get underway on I-5 in Thurston County, and that means road closures and detours for drivers.

RELATED: I-90 diamond interchange puts drivers on wrong side of the road

This new interchange is going in at the Marvin Road overpass on I-5 in the Hawks Prairie/Lacey area. There is so much new development on the north side of I-5 there, that the existing overpass can’t handle it.

That new development is being added to all the box stores and the shopping areas south of the freeway.

I’ll explain the diverging diamond in a minute, but first it’s important to let drivers know what’s coming.

“This is one of — if not the most — significant effect for traffic,” Washington Department of Transportation’s Doug Adamson said of this nearly two-year construction project.

Drivers can expect to see the detours show up in about a week. The biggest will be on the off-ramp from I-5 to Marvin Road.

“Anyone who is driving southbound off of I-5, wanting to turn left onto the Marvin Road, that will be temporarily closed,” he said. “Drivers will be detoured.”

Drivers will be forced onto a mile-long loop to get across the freeway and access the shops south of I-5.

Adamson said the overpass itself will be reduced to one lane in each direction.

There will be several full overpass closures toward the end of the two-year project as well. Those larger closures are being avoided on this project because the new interchange is using a lot of the existing structure.

“We don’t have to go out and tear down the existing one or build a temporary one and then build the permanent one alongside,” he said.

Diverging Diamond

As for the diverging diamond concept itself, this is the first one to be built in Washington. It puts drivers on the opposite side of the overpass while crossing the freeway. You essentially make half of a figure eight while crossing the freeway, if you’re going straight.

The major benefit is that drivers no longer have to wait for a light to make a left turn to access the freeway on-ramps. It nearly eliminates the possibilities of t-bone accidents with turning drivers being hit by on-coming traffic.

It might seem weird, but Adamson said drivers simply have to follow directions.

“You follow the signs and lines,” he said. “You come up to a stoplight, and you stop, then you’re directed.

The state is also preparing to build a second diverging diamond interchange where I-90 meets Highway 18 in Snoqualmie. Construction there is currently scheduled to begin in 2021.


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Construction starting on Washington’s first diverging diamond interchange