I-5 - Steilacoom-DuPont Rd. to Thorne Lane - Corridor Improvements

Project news

  • Guy F. Atkinson Construction, LLC was awarded the contract on May 17, 2018.
  • Design-Build contract will start mid-2018.
  • This project funded in the 2015 Connecting Washington transportation revenue package.

Needs & benefits

Why is WSDOT rebuilding the Berkeley Street and Thorne Lane interchanges and adding a lane in each direction on Interstate 5 between DuPont and Lakewood?
To relieve chronic traffic congestion and improve mobility along I-5 in the vicinity of Joint Base Lewis-McChord, WSDOT is rebuilding the interchanges at Thorne Lane and Berkeley Street and adding lanes to I-5.

The new interchanges are also grade-separated from the existing rail line, eliminating conflicts with trains. The new Thorne Lane interchange will incorporate roundabouts instead of signals which improve traffic flow and reduce the potential for collisions. 

Freeway capacity will increase with the addition of a lane in each direction between Steilacoom-DuPont Road in DuPont to Thorne Lane is Lakewood.

Auxiliary lanes are being added between Berkeley Street and Gravelly Lake Drive. These lanes are frequently used between closely spaced interchanges to allow for maneuvering of entering and exiting traffic, speed changes, weaving and truck climbing.

The End Result
The changes would reduce chronic traffic congestion through the JBLM corridor. New interchanges that incorporate roundabouts and grade separation from the rail line improves traffic flow and reduces the potential for collisions.

Project Benefits
Congestion Relief:

  • Additional capacity will improve traffic flow.
  • Northbound I-5 auxiliary lanes will be added between Berkeley Street and Gravelly Lake Drive, providing a supplemental lane for vehicles entering and exiting I-5.
  • Benefit-Cost Analysis Report (pdf 695 kb)


  • Grade separation from rail line
  • Builds roundabouts at Thorne Lane interchange. Roundabouts are shown to reduce injury crashes by 75 percent at intersections where stop signs or signals were previously used for traffic control.


  • The Interchange Justification Report was published September 2016.
  • On May 23, 2017, the U.S. Department of Transportation Federal Highway Administration issued a Finding Of No Significant Impact (FONSI) for the Interstate 5 JBLM Congestion Relief Project Environmental Assessment.
  • Design-Build contract awarded to Guy F. Atkinson Construction, LLC on May 17, 2018.
  • Design-Build contract will start mid-2018
  • Construction ends 2021


Financial Data for PIN 300503T
Funding Source Amount ($ in thousands)
2003 Gas Tax (Nickel Funding) $0
2005 Gas Tax (TPA) $0
Pre-Existing Funds (PEF) $0
CWA $332,477
Total $332,477

In July 2015, the Washington State Legislature included $495 million to fund the I-5 Mounts Road to Thorne Lane Interchange - Corridor Improvements project, as part of the Connecting Washington transportation revenue package. The project is funded through a 10 year period, from 2015-2025.


Chuck Meade, WSDOT Project Engineer

Cara Mitchell
WSDOT Olympic Region Communications

Small map for I-5 Steilacoom-DuPont Road to Thorne Lane project.