I-5 - Mounts Road to Thorne Ln I/C - Corridor Improvements

Project news

The I-5 Mounts Road to Thorne Ln  I/C Corridor Improvements project is broken up into 4 separate construction projects:

Why is WSDOT proposing to expand Interstate 5 through the Joint Base Lewis-McChord corridor?
Originally built in the 1950s, Interstate 5 is a national highway of strategic importance. It moves goods and people between two borders and links major cities. It is a critical corridor for military use. Over the past several years, traffic has increased along the entire I-5 corridor, and nowhere has that increase been more noticeable than in south Pierce County through the Joint Base Lewis-McChord (JBLM) vicinity.

The recommended improvements advanced forward from a joint planning study are:

  • Add one HOV lane to southbound I-5 from Thorne Lane to the DuPont area.
  • Add one HOV lane to northbound I-5 from Mounts Road to Thorne Lane.  
  • Rebuild interchanges at Thorne Lane and Berkeley Street using roundabouts.
  • Build a new local connector between Gravelly Lake Drive and Thorne Lane.
  • Build a bicycle/pedestrian path from Lakewood to DuPont.
  • Rebuild the Steilacoom-DuPont Road interchange.

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.

The End Result
The proposed changes would reduce chronic traffic congestion through the JBLM corridor. New interchanges that incorporate roundabouts instead of signals would improve traffic flow and reduce the potential for collisions.

Currently the only access into the Tillicum community is via I-5. The proposed Gravelly-Thorne connector would give the neighborhoods of Tillicum and Woodbrook access to Lakewood without using I-5. The proposed shared use path for bicycles and pedestrians would give Woodbrook and Tillicum residents a multi-modal option to commute to work or school.

Needs & benefits

Congestion Relief: Interstate 5 and surrounding local arterial roads would experience congestion relief and reduced delays. The Gravelly-Thorne connector would provide local access to Tillicum and Woodbrook neighborhoods without using I-5. The bicycle/pedestrian path gives commuters a multi-modal option to commute to work, and connects communities along the I-5 corridor.

Safety: New lanes that add capacity to the freeway will reduce collisions along the corridor. The new interchange at Thorne Lane, Berkeley Street on I-5 would improve traffic flow and enhance safety with the use of roundabouts.


  • In July 2015, the Washington State Legislature included $495 Million for the I-5 Mounts Road to Thorne Lane Interchange - Corridor Improvements project, as part of the Connecting Washington transportation-revenue package.
  • 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.

The overall project has been broken down into 4 construction projects:

  • Stage 1: I-5 - Mounts Road to Center Drive - Auxiliary Lane Extension - construction began in spring of 2017. Northbound I-5 was widened between Mounts Road and Center Drive for an auxiliary lane. The new extended auxiliary lane opened to traffic in Nov. 2017. Final striping occured in 2018. This project is complete. In a later stage of construction, this lane will be converted into a general purpose lane.
  • Stage 2: I-5 – Steilacoom –DuPont Rd. to Thorne Lane – WSDOT awarded the design-build contract to Guy F. Atkinson Construction, LLC for this project. The design build contract began mid-2018. This project will rebuild the Thorne Lane and Berkeley Street interchanges, and add a fourth lane to I-5 in each direction from Thorne Lane to just past JBLM's main gate. This fourth lane in each direction will be operated as an HOV lane once construction is complete.
  • Stage 3: I-5 – Mounts Road to Steilacoom-DuPont Road project - scheduled to begin in 2022. This project will build improvements between Mounts Road (Exit 116) and Steilacoom-DuPont Road (Exit 119). Improvements to the Steilacoom-DuPont Road interchange are being finalized.
  • Stage 4: Gravelly-Thorne Connector and Shared-Use path. Next steps for this project include right-of-way acquisition, final design and a competitive bidding process. Project construction will then occur beginning 2023.

All dates are preliminary and subject to change.


Financial Data for PIN M00100R
Funding Source Amount ($ in thousands)
2003 Gas Tax (Nickel Funding) $0
2005 Gas Tax (TPA) $0
Pre-Existing Funds (PEF) $0
CWA $494,400
Total $494,400


Bill Elliott
WSDOT JBLM Project Manager

Cara Mitchell
WSDOT Olympic Region Media Relations

I-5 Mounts Road to Thorne Lane I/C Corridor Improvements map