New interchange is open but westbound I-90 traffic likely won’t improve for months

The state’s newest interchange opened last Friday, at I-90 and State Route (SR) 18 in Snoqualmie, but it didn’t provide much relief during its first weekend.

I spent a lot of Sunday afternoon watching the westbound I-90 drive approaching the new Diverging Diamond Interchange (DDI) at SR 18. I wanted to see if the new design and increased capacity would help with that massive backup that always shows up on Sunday.

It did not, and it likely won’t for a few more months.

“Those Sunday afternoon commutes are still going to be challenging in the neighborhood,” Brian Nielson, the Regional Administrator for the Washington Department of Transportation (WSDOT), said.

We’re stuck with the backups until the construction finishes in a few months.

“We have to finish this widening first,” Nielson said. “The pinch point is really just south of the interchange, so until we finish that this fall, people should probably expect to see congestion through the interchange.”

Two lanes of westbound I-90 now exit to SR 18. That should help with that Sunday afternoon backup, once the widening is finished. That should help with the late merging.

What makes the DDI so unique is that when you exit to westbound SR 18, you turn immediately into the left lanes, the opposite lanes of traffic, through the interchange. It is very smooth but likely confusing the first time.

WSP’s goal with new interchange is to improve safety

For the Washington State Patrol, the goal is to improve safety.

“We’re thankful for this project, and we’re hoping to see a reduction of congestion,” WSP Lieutenant Pete Cozzitorto said. “We’d like to see traffic moving further, faster and allow people to get to where they want to be.”

Most drivers should expect a little learning curve using the DDI, but it shouldn’t last long.

“It’s gonna be new for a week or two, or the first couple times you go through there, but we expect, all in all, a better commuter experience with this interchange,” Cozzitorto said.

And hopefully, a lot less late merging, road rage, and frustration.

“I’m hopeful, with the two lanes exiting off of Interstate 90, that’s going to take some of the congestion off that right lane being stopped as people tried to exit in the past,” Cozzitorto said. “I think with this interchange having more capacity, we’re going to hopefully see a reduction in that congestion.”

WSDOT still has to do some paving and permanent striping at the interchange. There will be some closures to deal with as that work is completed.

Once the widening on SR 18 is done, there will be about four miles of two lanes south of I-90 to help ease up to the single lane at Tiger Mountain. That should greatly reduce the slowing on westbound 90.

The contractor is still on schedule to finish the widening this fall.

Chris Sullivan is a traffic reporter for KIRO Newsradio. Read more of his stories here. Follow KIRO Newsradio traffic on X.

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