Add/Drop Lanes

Local project partners decided in July 2008 that I-5 should have three through-travel (or general purpose) lanes in each direction through the Columbia River Crossing project area. They also agreed additional lanes should connect some interchanges to allow for safe merging or exiting the highway. Engineers call these lanes auxiliary lanes; the CRC project often uses the term “add/drop” lanes.

In August 2010, the Project Sponsors Council unanimously recommended a permanent 10-lane bridge with full safety shoulders. This allows both directions of the replacement I-5 bridge to contain three through lanes and two add/drop lanes, making a total of 10 lanes on the bridge.

What is an add/drop lane?

An auxiliary or add/drop lane connects two or more highway interchanges. These lanes improve safety and reduce congestion by providing space for cars and trucks entering the highway to speed up before merging into traffic and to slow down after diverging out of traffic. One way to identify an add/drop lane is by the “exit only” sign posted on the highway.

Illustration without an add/drop lane

Illustration with an add/drop lane

Why are add/drop lanes necessary?

Improved Safety: An average of 400 crashes a year occur along I-5 in the five-mile project area, a collision rate two times higher than similar highways in Oregon and Washington. With seven closely-spaced interchanges, most highway entrances in the project area require vehicles to merge into a through-lane quickly upon entering the highway. These conditions lead to crashes. The Hayden Island interchange on-ramp to northbound I-5 is the highest crash location on I-5 in Oregon. Add/drop lanes would provide drivers with more space to merge safely.

Reduced Congestion: Congestion within the project area could be reduced by nine hours a day compared to No Build conditions.

Better Operations: Add/drop lanes can serve as a local arterial between Marine Drive, Hayden Island and Vancouver; provide better connections for freight from I-5 to the ports; and result in less spillover traffic to city streets.

How did the project determine the right number of add/drop lanes?

The CRC Project Sponsors Council (PSC) recommended in March 2009 that the replacement bridge should include three add/drop lanes in each direction. The recommendation was based on technical information and public comment. As part of the recommendation, the PSC also recommended creation of a Mobility Council to manage long-term performance of the Columbia River crossings.

In August 2010, the PSC unanimously agreed on a set of recommendations to the governors of Washington and Oregon for moving ahead with development and construction of the I-5 bridge project. The recommendations include designing a replacement bridge with three through lanes, two add/drop lanes and full safety shoulders in each direction, making a 10-lane facility. These recommendations came as result of work completed by an integrated group of staff members from all agencies represented on the council, as well as the ports of Portland and Vancouver. The analysis found a 10-lane bridge performs similarly to a 12-lane crossing.