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

Project news

  • COVID-19 construction effects
  • The new Thorne Lane interchange is open in a temporary configuration. Read the latest blog that covers what to expect during the detours and ramp closures. This new video shows how the new I-5 Thorne Lane interchange will operate once construction is complete.
  • The second step to raising I-5 at Berkeley Street occurred overnight on Oct. 24 when southbound I-5 lanes shifted to almost their final alignment. Read on our blog how crews are raising I-5 14-feet at Berkeley Street.
  • A temporary regulatory speed limit reduction from 60 mph to 50 mph is in place on both directions of I-5 from milepost 121.9 to milepost 124.1. The temporary speed limit reduction will remain in place until June 2021. 
  • Starting Monday, March 16, a one-block section of Union Avenue is closed between Spruce Street and Thorne Lane. The around-the-clock closure will be in place while crews raise the profile of Union Avenue over 23 feet so the street can connect with the new Thorne Lane overpass. During the closure, drivers will detour using Thorne Lane, Portland Avenue and Berkeley Street. The WSDOT blog has more information about the I-5/Thorne Lane interchange.
  • View the latest ramp and lane closure schedule on the Pierce and Thurston County travel planner webpage.
  • For real-time updates, follow @wsdot_tacoma on Twitter, visit the travel alerts page and download the mobile app.

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. Once construction is finished, the new lane in each direction on I-5 will open as a high occupancy vehicle lane. 

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:

  • Adding one HOV lane in each direction of I-5 between Steilacoom-DuPont Road in DuPont and Thorne Lane in Lakewood will add additional capacity and improve traffic flow.
  • Auxiliary lanes are being added on northbound I-5 between Berkeley Street and Gravelly Lake Drive, and on southbound I-5 from Gravelly Lake Drive to Thorne Lane, and from Berkeley Street to JBLM Main Gate, providing a supplemental lane for vehicles entering and exiting I-5.
  • Benefit-Cost Analysis Report (pdf 695 kb)


  • The new interchanges are grade-separated from the existing rail line, reducing conflicts with trains.
  • 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.

What to Expect

  • Narrowed and shifted lanes on I-5 and ramps in the project limits.
  • Daytime shoulder closures are allowed on both directions of I-5 from 9 a.m. to 3 p.m. in the project limits.
  • Single and double lane closures on northbound and southbound I-5 each week night from 8 p.m. to 5 a.m. each following day.
  • Overnight I-5 ramp closures of 41st Division Drive, Berkeley Street/Jackson Avenue, Thorne Lane/Murray Road and Gravelly Lake Drive. Typically, no two adjacent ramps will be closed overnight simultaneously.
  • Occasional weekend ramp closures and one extended closure of the Berkeley Street on-ramp to northbound I-5. 
  • Extended closure of Union Avenue at Thorne Lane starting mid-March 2020.
  • Access to Joint Base Lewis-McChord and Madigan Army Medical Center will be maintained.


  • 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 began mid-2018.
  • October 2018 - construction began at the Berkeley Street interchange.
  • February 2019 - construction begins at the Thorne Lane interchange.
  • April 2019 - widening of Interstate 5 begins.
  • Winter 2020 - new Berkeley Street interchange opens.
  • Fall 2020 - new Thorne Lane interchange opens.
  • Summer 2021 - traffic in final configuration
  • Summer 2021 - final paving and striping, and construction ends.


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 $243,098
Total $243,098

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.


Brian Whitehouse,
WSDOT Project Engineer

Cara Mitchell
Media Relations
WSDOT Olympic Region