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The already congested drive on State Route 18 to Interstate 90 is going to get a lot worse as construction ramps up this week.

The truck climbing lane or passing lane has already been shut down, which has caused long delays every morning on eastbound SR 18 over the Tiger Mountain summit toward I-90. Now, the lanes are going to be reduced even more, creating one lane of alternating traffic at times on that stretch of highway. .

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“They’re going to have single lane alternating traffic at the interchange and also near the Raging River, they’re going to also do some single lane alternating traffic,” the Washington Department of Transportation (WSDOT)’s Adrienne Hatmaker said.

Glen Vandevort is the assistant project manager for Aecon Construction, which is the contractor on the job, and he said that he’s seen backups all the way to the top.

“It does get backed up pretty bad,” Vandevort said. “I’ve seen it backed up to the top of Tiger Mountain, and I would expect to see similar backups going forward through the duration.”

The duration of the construction is until late 2024 or early 2025.

So what is happening that will require all this temporary congestion?

“Widening SR 18 from the bass of Tiger Mountain to the I-90 and then the diverging diamond interchange,” Vandervort said. “We’re building six or seven new drainage ponds to collect stormwater and treatment facilities to also help with all that.”

And they are improving six fish passages along the route as well.

When this project is done, there will be a new diverging diamond interchange at I-90 and SR 18 and two full lanes in each direction of SR 18 from I-90 to the base of Tiger Mountain.

This will be the state’s second diverging diamond interchange. The first one was installed in Lacey a few years ago. They are designed to eliminate crossing traffic at freeway crossings, so you end up driving in the opposite lane of traffic.

Once that is done, another project will widen SR 18 the rest of the way to Issaquah-Hobart Road.

Why is this important?

There is so much traffic now using this corridor that it just can’t handle it, and anyone who has driven westbound on I-90 on a Sunday afternoon can attest that the sudden complete stop at SR 18 is dangerous.

“You come around the corner on I-90, and all of a sudden, you’ve got that right lane backed up for stopping traffic,” Hatmaker said. “That’s really what we’re trying to get past is keeping people flowing through there, keeping it safer both on I-90 and getting people where they need to go.”

The new interchange should also eliminate all the big rig rollovers created by drivers going too fast.

“The ramps they’re building are going to have a wider turning radius for trucks,” Hatmaker said. “That has been a pinch point in the past where trucks making those corners are coming too fast, so that’ll be a great improvement too.”

Hatmaker knows this is going to be a difficult two years of construction, but she said it will be worth it.

“Just be aware that you’re driving past work zones and keep yourself safe, keep the workers safe, and know that in the end, we are going to have a widened two-lane each direction highway,” Hatmaker said. “It’s going to be a little tight in the meantime, so just add time to your travels.”

Check out more of Chris’ Chokepoints.