State: Three Milestones Push I-90/SR 18 improvement project to next phase of development

On September 19, 2019, the Washington State Department of Transportation said three major advancements had pushed the I-90/SR 18 interchange improvement project into the next phase of development, including decisions about the interchange, an improved on-ramp to westbound I-90, and a new weigh/inspection station.

Interchange Design Alternative Officially Chosen

For about almost four years we’ve been reporting that a diverging diamond interchange was the option being pursued, but now WSDOT has made it the official preferred design alternative to improve the 90/18 interchange. This lower-cost option is said to move more cars and reduce conflict points among vehicles compared to traditional diamond interchanges. You just have to get used to driving on the wrong side of the road while using it.

“There are more than 60 diverging diamond interchanges around the country and they consistently show collisions decrease,” said WSDOT Engineering Manager John Chi. “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.” 

WSDOT will be conducting interchange design, environmental investigation and analysis, along with more community engagement, through 2020.

Construction is scheduled to begin in 2021 with the new interchange opening in 2023. 

The state’s first diverging diamond interchange is under construction on the State Route 510 intersection with I-5 near Lacey. 

New Truck Inspection/Weigh station

As we reported last month, WSDOT is taking a fresh look at locations for a WSP freight inspection station farther east of North Bend. WSDOT said following guidance from local agency, county, and city representatives and extensive community feedback, it will work with the Washington State Patrol to continue the search for other viable sites.

The $150 million interchange improvement and weigh station project is funded through the 2015 Connecting Washington transportation program. 

New On-ramp to Westbound I-90 from Snoqualmie Parkway

Construction will officially begin next month to upgrade the existing ramp from Snoqualmie Parkway to westbound I-90. WSDOT awarded the contract to Road Construction Northwest, Inc. of Renton. The decommissioned westbound weigh station property will be used to create a second on-ramp lane for highway users.

“We are very pleased that this much-needed temporary designated on-ramp project is now moving forward and is projected to be completed before the end of the year,” said Snoqualmie Mayor Matt Larson. “It will provide immediate morning traffic congestion relief three years prior to original projections.”

The modifications include:

  • A dedicated right turn lane with no traffic signal for southbound Snoqualmie Parkway to westbound I-90 drivers, along with a dedicated ramp lane. 
  • A dedicated ramp lane for eastbound SR 18 drivers turning west onto I-90.
  • A retimed traffic signal for SR 18 to westbound I-90 to provide a longer green light for drivers. 
  • Traffic meters for both ramps.

The portion of the on-ramp that directed freight haulers to a weigh station will be rebuilt and re-striped and the old weigh station building removed. Snoqualmie Parkway drivers will have a free right turn into a dedicated turn lane that will take them through the old weigh station area before merging them onto westbound I-90. Drivers on SR 18 will also have a dedicated ramp lane.

WSDOT says the additional ramp lane and retimed signal will result in shorter backups and improved traffic flow at this intersection.

The $1.2 million project is funded by WSDOT, the cities of Snoqualmie, Covington, Maple Valley, and the Snoqualmie Tribe. It is expected to be finished by the end of December. The on-ramps will be modified again and connected to the new I-90/SR 18 interchange prior to its opening in 2023. 

New WB ramp to I-90 from Snoqualmie Pkwy

Comments are closed.


  • What makes the lanes “dedicated”? A barrier? Per the diagram, the lanes only split 1/3rd of the way in. So if no barrier, i can see the Parkway lane filling with SR18 drivers if the metering lights cause any kind of backup…

  • Has anyone read about the detour for i90 westbound from the parkway and hwy 18 during this initial construction of the temporary westbound on ramp? It appears that the detour will have all traffic from the parkway and 18 going westbound taking the I90 eastbound on ramp then exit at the casino then onto the westbound on ramp from there. Here is the link to the plan. If this plan is happening seems like a few ugly months of traffic for anyone heading westbound in the morning or evening.—Detour-Plan-PDF

    1. I *think* it’s only northbound 18 that will detour on eastbound 90. The former weighstation is *supposed* to be the detour for Sno. Parkway.

      1. Looking at the diagrams, locations of detour signs, and the directions indicated on the signs from the view point of drivers viewing them, YES this does indicate drivers on the Parkway trying to go West on I-90 will follow the detour onto I-90 East to exit 27, and then back onto I-90 Westbound. This will be a big disruption during construction. And will also cause traffic load to reroute to 202/Fall City/Preston, which will also back up.

  • Don’t worry, there’s a couple of guys with clipboards in sandals and socks who ride their bikes to work that have got this all figured out.

  • I am all for this except for the metering part! Metering is going to cause more problems then it solves. People just need to be responsible and freaking speed up to merge and let other people merge at speed. Too many people merge barely at or below the speed limit. We have a quarter mile to speed up to 70 to merge. To not do so is a severe safety issue. Metering will compound this issue. Again WasDoT Traffic control vs traffic flow policies…

  • Living Snoqualmie