State to begin work on diverging diamond interchange at I-90, SR 18

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  (Courtesy Aecon via WSDOT)

A major project is set to get underway this fall in an effort to get traffic moving at one of the state's busiest interchanges at Interstate 90 and State Route 18 in Snoqualmie Valley.

In addition to creating the second diverging diamond interchange in Washington state, the project will also add two lanes on SR 18 between the interchange and Deep Creek.

The $188 million project will begin in November and is scheduled to be finished in 2025.

Here's what will change with the new project:

  • Creating a diverging diamond interchange
  • Widening two miles of SR 18, providing two lanes in each direction
  • A new 2-lane bridge over Raging River next to the existing bridge
  • Two new 4-lane bridges one at Deep Creek and one at Lake Creek
  • Replacing fish passage barriers with relocated, open channel streams
  • Improving habitat connectivity for wildlife including deer and elk

"We are investing in a transportation corridor that’s critical for moving travelers and products across our state, while at the same time improving riparian and wildlife habitat and access," Washington Gov. Jay Inslee said at a news conference on Thursday. "Together with cleaner fuels and more electric vehicles on the road, we are moving in the right direction for our people, our economy and our environment."'

What is a diverging diamond interchange (DDI)?

Washington state currently has one DDI on I-5 at Marvin Rd. in Lacey.

This style of interchange allows through-traffic and left turns to happen at the same time which eliminates the need for a separate left-turn phase:

"A diverging diamond interchange is a proven way to move more people and reduce the potential for collisions by reducing the number of conflict points between vehicles. There are fewer traffic lights and more opportunities for free turns to enter or exit the interstate."

The number of conflict points drops from 26 down to 14 which reduces the potential for collisions.

"The widening project together with the diverging diamond interchange will keep traffic moving more freely, preventing backups at peak travel times." James Harper, WSDOT project engineer said.