SNOQUALMIE, Wash. — Editor's note: the above video was originally published in April 2019.
Crews will construct a diverging diamond interchange at the State Route 18/Interstate 90 interchange to improve traffic flow and reduce congestion instead of a traditional diamond interchange.
“They consistently show collisions decrease,” Washington State Department of Transportation Engineering Manager John Chi said in a statement Thursday. “Our traffic models show this new interchange will prevent the backups that occur every day, and get more highway users on and off the highway in a safe and efficient manner.”
The diverging diamond is a proven way to move more cars and reduce potential collisions by limiting conflict points.
Motorists, get used to the idea of opposing traffic passing on the right instead of the left during these interchanges. That allows through traffic and left turns to happen at the same time, which means there isn't a need for a separate left turn light.
The new interchange is scheduled to open in 2023.
The first diverging diamond interchange, which is under construction, will be on State Route 510 at I-5 near Lacey. The Marvin Road overpass will be closed to all vehicle and pedestrians from 10 p.m. Friday, September 27 to 5 a.m. Monday, September 30 while crews work on the interchange project.
Another project aimed at improving the SR 18/I-90 interchange will be upgrading the existing ramp from Snoqualmie Parkway to westbound I-90 with a second on-ramp lane beginning next month.
There will be a dedicated right turn lane with no traffic signal for drivers going southbound to westbound as well as a dedicated ramp lane. There will also be a dedicated ramp lane for eastbound drivers turning west onto I-90.
The traffic signal for SR 18 to westbound I-90 will have a longer green light for drivers, and traffic meters will be installed for both ramps just before the ramps merge.
Lastly, WSDOT is also looking for new viable locations for a Washington State Patrol freight inspection station farther east of North Bend as a part of the state's Connecting Washington plan.
The westbound weigh station at the I-90/SR18 interchange is now closed for improvements.
Freight mobility will be improved, multimodal transportation options such as walking and bicycling will be safer, and even salmon recovery efforts will be enhanced through the Connecting Washington project.