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


March 2017

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.

Project Benefits

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

What is the project timeline?

  • Dec. 28, 2016 - Contract awarded to Scarsella Bros, Inc.
  • February 2017 - construction begins
  • Fall 2017 - construction complete

Financial Information

Project signage will reflect the cost of construction engineering, project bid award and sales tax.

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.

How can I get more information?
Jon Keeth
Construction Project Engineer

Claudia Bingham Baker
WSDOT Communications Manager

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