The Washington State Department of Transportation (WSDOT) has completed another stage of State Route 18 construction at Interstate 90.

The new diverging diamond interchange under the I-90 bridges — only the second in the state — opened July 25 after a week-long closure on that part of SR 18.

A diverging diamond interchange, where both directions of traffic temporarily cross onto the left side of the road, is safer than the previous configuration, according to WSDOT.

“Vehicles no longer have to turn left in front of oncoming traffic, allowing for fewer traffic-signal phases,” a news release says. “The diverging traffic pattern also improves safety by reducing the number of locations in the interchange where vehicles could potentially collide with each other or a pedestrian.”

The interchange is part of larger work on the two-mile stretch of SR 18 from I-90 to Deep Creek that is expected to finish this fall.

According to WSDOT’s website, the project is also adding two lanes to the highway, a traffic signal-controlled U-turn just west of the interchange, a new two-lane bridge over Raging River and two new four-lane bridges. It also replaced fish passage barriers with relocated open channel streams and improved habitat connectivity for wildlife, including deer and elk.

Construction continues on SR 18 as WSDOT works to widen the highway between I-90 and Issaquah-Hobart Road SE. That project will occur in two phases and add two lanes to the highway, as well as introduce other safety improvements. It is currently in the pre-construction phase, with construction not starting until at least 2027 and lasting into the 2030s.

For real-time traffic information, download the WSDOT app or visit WSDOT’s statewide travel map.

Snoqualmie Mayor Katherine Ross (left) and Washington State Sen. Victoria Hunt at the opening of the State Route 18 diverging diamond interchange in Snoqualmie, July 25, 2025. Photo courtesy of Marcus Humberg/Washington State Department of Transportation

The new diverging diamond interchange on State Route 18 in Snoqualmie. Photo courtesy of Marcus Humberg/Washington State Department of Transportation