I-5 - Mounts Rd. to Center Dr. - Auxiliary Lane Extension - Complete September 2018

Project news

  • This project is now complete.
  • Contractor crews striped southbound I-5 to its final configuration. Travelers who use the Center Drive on-ramp to southbound I-5 now merge left twice as they proceed to southbound I-5. More information about this can be found on the WSDOT Blog.
  • The northbound I-5 auxiliary lane extension between Mounts Road and Center Drive opened to traffic in November 2017.
  • 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 HOV 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.
  • Summer 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 - $11,742
  • Total - $11,742

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.


Lone Moody
Construction Project Engineer

Cara Mitchell
WSDOT Communications

Mounts Rd to Center Dr Aux lane extension small map