The Old Highway 30 Bridge, designed by Caldwell City Engineer F. H. McConnel, was built in 1922 by the American Bridge Company. This subsidiary of United States Steel, who also built famous structures like the Woolworth Building, the Chrysler Building, the Empire State Building, the Sears Tower and the San Francisco Bay Bridge.

At that time the bridge was called the Boise River and Canal Bridge, and at construction was part of the Idaho Pacific Highway that later became U.S. Highway 30. It continued to be called U.S. Highway 30 until the early 1960s when the nearby Interstate was constructed and the southeast portion of the roadway was renamed Plymouth Street, while the northwest side was renamed Old Highway 30.

The bridge allows access across the river and canals. It spans 388 feet across the Boise River and consists of a three-span through-truss. The bridge is beyond its functional lifespan and needs to be replaced. The existing structure is proposed to be converted into a pedestrian pathway as part of Caldwell's Boise River Greenbelt.

Old Highway 30 Bridge
Old Highway 30 Bridge

Why Does the Bridge Need to be Replaced?

The Old Highway 30/W. Plymouth Street Bridge is located off Interstate 84 Exit 26 in Caldwell Idaho. The existing one-lane bridge crosses the Boise River, provides local access to the river, and surrounding neighborhoods. It is also a link between Caldwell and the Middleton area. This route has been used as a bypass to Interstate 84.

The current steel truss bridge is a three-span, 388-foot long structure constructed in 1922 and carries 1,350 vehicles a day, including buses and trucks. The existing bridge needs to be replaced due to its narrow width and poor, aging condition.

The general work will bring about a new bridge structure, bridge approach roadway, pathway relocation, and rehabilitation/retrofit of the existing bridge for pedestrian use. The improvements will provide a safe route for vehicles, bicycles and pedestrians.

What Will Happen to the Old Bridge?

It is planned to convert the existing bridge into a bicycle and pedestrian pathway. Proposed improvements include a new concrete deck and pedestrian railing. This approach will also allow separation of pedestrian/bicycle traffic from vehicular traffic providing for a safer pathway system.

Community Involvement

Canyon Highway District 4 and the City of Caldwell will use multiple communication tools and innovative technologies to gather public input and assess concerns of the local community.

Future opportunities for community involvement include:

  • Input Interviews - for example, City of Caldwell, Irrigation Districts, Flood Control Districts, Idaho Rivers United, etc.
  • Web and Social Media Support
  • Project Update Newsletters
  • Open House Public Involvement Meetings

An Open House Public Meeting was held in July 2016 to introduce the project to the public and gain valuable information from the residents in the area. Comments were accepted through August 7, 2016. The following are links to the bridge & road alignment concepts and proposed typical sections presented at the public meeting:


Old Highway 30 Bridge Concepts 1.91 MB 2805 downloads



Stephen Waldinger
Forsgren Associates, Inc.
(208) 342-3144

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