The Washington State Department of Transportation (WSDOT) has entered the final stage of the $188 million Interstate 90/State Route 18 interchange expansion and improvement project.
The interchange is one of the busiest in the state and, before the recent upgrade, was often backed up at peak times and on busy travel weekends. The current phase involves the final paving and is expected to be completed by the end of the summer, according to the WSDOT website.
The comprehensive project involved several stages of construction which included significant upgrades and alterations to the traffic patterns, roadways, infrastructure and natural environment.
Aecon Pacific Northwest Inc. is serving as the contractor.
Traffic Flow, Safety Improvements
Washington State Department of Transportation photo
The traffic interchange at the juncture was converted to a diverging diamond pattern. This new pattern is safer because it prevents left turns across oncoming traffic and reduces the number of spots where vehicles can collide or encounter pedestrians and bicycles.
This new interchange reduces the number of potential conflict points from 26 to 14, which, in turn, reduces the potential for collisions by nearly 50 percent, according to the WSDOT website.
The WSDOT also improved safety on the off-ramp from I-90 to SR 18 by widening the eastbound I-90 off-ramp and extending “exit only” striping on the approach to the exit.
Two new active warning signs on eastbound I-90 were installed to alert drivers of stopped traffic ahead, and additional warning signs with flashing yellow beacons were installed south of the Southeast 104th Street intersection to signal to drivers that vehicles are entering and exiting the highway at the intersection.
In partnership with the city of Snoqualmie, the WSDOT altered the lane configuration on southbound Snoqualmie Parkway to make the right lane a right-turn-only lane to the westbound I-90 on-ramp, according to the WSDOT website.
Further improvements involved optimizing the traffic signal timing for maximum efficiency, installing “no U-turn” signs, and adding “thru traffic keep left” signs to prevent congestion and keep drivers in the proper lanes for their destinations.
In addition to the interchange upgrade, two miles of SR 18 were widened between Deep Creek and I-90, creating two lanes of traffic in each direction to improve flow. New guardrails and traffic loops were also installed.
Infrastructure Upgrades
Washington State Department of Transportation photo
In addition to traffic flow improvements, the project encompassed significant upgrades and additions to infrastructure.
These included the construction of a new four-lane bridge over Lake Creek and a new two-lane bridge over Deep Creek. A complete drainage plan was implemented, and this included two culverts that enable runoff under both the eastbound on-ramp and westbound off-ramp on I-90, according to the WSDOT website.
An old weigh station near the westbound I-90 on-ramp was removed, and a top layer of asphalt was added to the entire project area.
Environmental Improvements
Environmental factors were considered in the scope of this project; the two new bridges referenced above are key examples.
The bridge designs allow fish to pass under SR 18 at both Deep Creek and Lake Creek. The new system restored access to 13 mi. of stream habitat which is essential for salmon, steelhead and other species. The bridges also allow wildlife to pass safely under the highway without encountering traffic.
The project enhanced the Deep Creek watershed, opening access to 11.2 mi. of stream habitat, .75 acres of spawning territory, and 2.34 acres of rearing habitat, according to the WSDOT website.
Final Stage
Washington State Department of Transportation photo
The project commenced in 2022 with the first phase of construction breaking ground in April of 2023. All work is expected to be completed in the coming weeks.
The final stages of paving were delayed due to inclement weather, but starting in April, three extended weekend closures enabled crews to apply a top layer of asphalt. As of May 11, all lanes of east and westbound SR 18 between Southeast 104th Street and Deep Creek have been paved, along with the eastbound I-90 off-ramp.
The double westbound on- and off-ramp at the interchange of SR 18 and I-90 were among the last items to be completed. In addition, the right-turning ramps in that section of the roadway will undergo final paving in this last stage.
Washington State Department of Transportation photo
The total project budget is $188 million; $5 million came from the Connecting Washington Transportation Package, and $183 million came from funds generated through the state gas tax, according to the WSDOT website.
