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

UPDATE: Travelers who use I-90 and the interchange at State Route 18 should prepare for slowdowns and intermittent lane closures starting Jan. 30 as construction for a diverging diamond interchange ramps up.

Closure information from WSDOT:

From 8 a.m. to 3 p.m. daily Monday, Jan. 30 to Saturday, Feb. 4, workers will close the westbound SR 18 truck climbing lane for geotechnical drilling. Crews are preparing to widen SR 18 from Issaquah-Hobart Road to just north of the Tiger Mountain summit and could begin as early as 2025.

Contractor crews will place traffic advisory signs throughout the I-90/SR 18 project area.

  • From 7 a.m. to 3 p.m. each day, Monday, Jan. 30 to Wednesday, Feb. 1, crews will conduct short duration left lane closures on I-90 in both directions near the interchange.
  • From 7 a.m. to 3 p.m., Tuesday, Jan. 31 and Wednesday, Feb. 1, expect short duration shoulder closures on SR 18, in both directions, between the interchange and about two miles to the south.
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  (Courtesy Aecon via WSDOT)

Original story:

A major project is set to get underway 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 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.