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Speed limits to lower on SR 18 near Tiger Mountain during interchange construction

WSDOT is building a diverging diamond interchange between I-90 and SR 18 near Snoqualmie, and construction means drivers will need to slow down.

SNOQUALMIE, Wash. — The speed limit on state Route 18 near Tiger Mountain will be lowered for more than a year starting Monday. 

Speed limits will be 30 miles per hour between the Interstate 90 interchange and just south of Southeast 104th Street and 45 miles per hour between just south of Southeast 104th Street and just north of the Tiger Mountain summit, according to the Washington State Department of Transportation (WSDOT). The speed limit will then return to 55 mph. 

The temporary speed reduction is expected to last through late 2024 while crews rebuild the I-90/SR 18 interchange into a diverging diamond interchange and widen SR 18. 

WSDOT said the speed reduction was necessary to keep workers and drivers safe while construction is underway.

“Reducing the speed limit through our project area will help people navigate those traffic shifts through each stage of construction,” WSDOT Project Engineer James Harper said in a statement.

New speed limit signs will be installed. 

WSDOT plans to work with Washington State Patrol to enforce the new speed limits. Fines will be doubled in the work zone. 

I-90 and SR 18 are undergoing construction to build a diverging diamond interchange as of November 2022.

A diverging diamond interchange is “a proven way to keep traffic flowing and improve safety by reducing the potential for collisions,” according to WSDOT. The new structure on I-90 and SR 18 is expected to reduce the number of conflict points from 26 to 14, which reduces the potential of collisions by 50%. This structure will be very similar to the one built on I-5 and Marvin Road in Lacey.

The construction officially began in April 2023. It started with removing trees in the project area and began the highway widening. 

There is one lane going in each direction of SR 18, which is shifting traffic to the east side of the road to create a safe work zone. For those heading west, be cautious of drivers turning on Southeast 104th Street during peak travel hours.

Some additions to the project include a four-lane bridge over Lake Creek on SR 18, which is just south of I-90, and fish relocation to remove a blocking culvert. These changes will help improve “habitat connectivity” for wildlife within Washington.

WSDOT has been doing community outreach since 2018 to help select an appropriate design for the structure and to boost local awareness of the project. Later this year, there will be an open house to look at the developments of the project. 

The diverging diamond interchange is set to open in 2025.


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