Budget Information

I-5 - Mounts Rd. to Center Dr. - Auxiliary Lane Extension

Project news

  • The northbound I-5 auxiliary lane extension between Mounts Road and Center Drive is now open to traffic.
  • Crews have installed a temporary overhead sign on the Center Drive overpass. A new sign bridge will be installed in the coming weeks.
  • Northbound I-5 will be restriped during the week of March 12 during overnight hours if the weather allows. All work is dependent on dry weather.
  • Southbound I-5 will be restriped to its original 12 foot lane configurations with a widened shoulder once the weather allows.
  • Funding for this project is included in the I-5 Mounts Road to Thorne Lane I/C Corridor Improvements project.

What is an auxiliary lane?
An auxiliary lane is a supplemental or extra lane between interchanges. They are frequently used between closely spaced interchanges to allow for maneuvering of entering and exiting traffic, speed changes, weaving and truck climbing.

There is an existing auxiliary lane along northbound I-5 between the Center Drive (Exit 118) and Steilacoom-DuPont Road (Exit 119) interchanges. It is about 0.4 miles in length.

Why is WSDOT extending the auxiliary lane on northbound Interstate 5 between Center Drive and Steilacoom-DuPont Road one and a half miles south to Mounts Road?
Traffic volumes on Interstate 5 between Mounts Road and Steilacoom-DuPont Road have increased substantially over the past 10 years. Traffic flow through this 2-mile area is subject to several pressures, including the uphill grade from the Nisqually River valley, entering traffic from Mounts Road, trucks using the weigh station, exiting and entering traffic at the Center Drive interchange and exiting traffic to the Steilacoom-DuPont Road interchange and JBLM’s DuPont Gate.

By extending the auxiliary lane south about 1.5 miles to start at the Mounts Rd. bridge, trucks approaching the weigh station along with entering and exiting traffic will use this lane while the majority of traffic uses the adjacent three through lanes.

To accommodate the auxiliary lane, northbound I-5 will be widened into the center median. The final lane configurations will include three 11-foot-wide lanes, one 12-foot-wide lane and 8 feet on the inside shoulder.

This work has independent utility, meaning it provides benefits to the traveling public independent of other I-5 - Mounts Road to Thorne Lane I/C Corridor Improvements project elements.

The End Result
The extended auxiliary lane will ultimately become part of the fourth general purpose lane that is being built in the I-5 Mounts Road to Thorne Lane I/C Corridor Improvements project.

Needs & benefits

  • Reduce congestion by providing a supplemental lane for trucks and traffic entering and exiting I-5.
  • Higher center median safety barrier.


  • Dec. 28, 2016 - Contract awarded to Scarsella Bros, Inc.
  • February 2017 - construction began.
  • November 2017 - extended auxiliary lane opened to traffic.
  • Spring 2018 - construction complete.


Financial Data for PIN 300502T
Funding Source Amount ($ in thousands)
2003 Gas Tax (Nickel Funding) $0
2005 Gas Tax (TPA) $0
Pre-Existing Funds (PEF) $0
CWA $11,249
Total $11,249

This project is included in the $495 Million for the I-5 Mounts Road to Thorne Lane Interchange - Corridor Improvements project, as part of the Connecting Washington transportation-revenue package.


Jon Keeth
Construction Project Engineer

Claudia Bingham Baker
WSDOT Communications Manager

Mounts Rd to Center Dr Aux lane extension small map