State Route 18 closed over Tiger Mountain through Monday

There is light at the end of the highway on the State Route 18 (SR 18) widening project between Deep Creek and I-90, a project years in the making.

On Monday, October 20, two lanes will open along this 2-mile stretch in both directions of SR 18, bringing much-needed relief to the 24,000 daily commuters.

Leading up to Monday’s grand reopening, however, is a four-day closure on SR 18 between I-90 and Issaquah-Hobart Road to put the final touches on this project.

David Rasbach, with the Washington State Department of Transportation (WSDOT), spoke with KIRO Newsradio.

“Well, we’re going to be shifting SR 18 into its new configuration, which will be two lanes in each direction, between I-90 and Deep Creek,” David Rasbach, spokesperson for the Washington State Department of Transportation (WSDOT), told KIRO Newsradio. “So this is a pretty exciting milestone. The first phase of this project was opening the diverging diamond interchange, which we did in July. And this is really the second big part of that project, widening state route 18 for two miles in each direction, and it’s all about improving traffic flow through that corridor and improving safety.”

Rasbach explained why this project is such a big deal.

“There’s a pinch point located a couple hundred feet south of I-90,” he said. “And basically, that’s taking all traffic that wants to travel westbound on SR 18 towards Auburn, and funneling it down to one lane. So that includes traffic coming from Snoqualmie and Snoqualmie Parkway. That includes two lanes of traffic coming from westbound I-90 and two lanes of traffic coming from eastbound I-90. It’s all funneling down into one lane now.”

The bottleneck is creating heavy traffic backups.

“What we’re seeing during busier travel times, such as afternoon commutes, Sunday afternoons, is backups, and that congestion kind of stretches through the DDI (diverging diamond interchange), and it stretches up to I-90 and even onto mainline I-90,” Rasbach said. “So when we remove that pinch point, the diverging diamond will be able to function like it was intended to do, and move traffic through that corridor and down westbound SR 18.”

State Route 18

Future State Route 18 closures

This project isn’t quite finished yet. In the weeks ahead, WSDOT will once again have single-lane closures along this new stretch of SR 18 to finish minor details. Backups are expected to be minimal during these lane closures now that a second lane has been added. Dates for these lane closures have yet to be announced.

And then the cherry on top will be a final top layer of pavement.

“It’ll be quite a bit of pavement,” Rasbach shared. “We have to put a top layer of asphalt over that entire section of SR 18, through the diverging diamond itself, and on the on and off ramps to I-90. That asphalt needs prime weather conditions. It can’t be raining. It needs to be a certain temperature, so it cures properly. So we’re at a point in 2025 where we’re just not going to see that weather anymore. So we are going to finish applying that top layer of asphalt in 2026 once the weather warms up.”

In the meantime, during this four-day closure, WSDOT has a marked detour, and it emphasizes that freight needs to adhere to it.

“We’re trying to push all freight traffic off of Issaquah-Hobart road and Issaquah city streets, which are not suitable for freight, for semi trucks, basically. We’re asking them to stick to the interstates, I-405, I-90, State Route 169,” Rasbach said.

Drivers should anticipate heavy volumes along these routes through Monday morning when all lanes of SR 18 reopen.

Nate Connors is a traffic reporter for KIRO Newsradio. Follow him on X. Read more of his stories here. Submit news tips here.

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