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.

The CRC Project Sponsors Council recommended in March 2009 that the replacement bridge should include three add/drop or auxiliary lanes in each direction. This allows both directions of the replacement I-5 bridge to contain three through lanes and three add/drop lanes, making a total of 12 lanes on the bridge.

What is an add/drop lane?

An add/drop lane connects two or more highway interchanges. Add/drop 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. 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 Project Sponsors Council recommended the number of add/drop lanes 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. The project is continuing technical work on bridge design, environmental analysis and interchange design.