SH-55 Rainbow Bridge

Nearly a century after its construction, the current Rainbow Bridge no longer meets ITD standards for state highway bridges. Despite regular maintenance and major repairs, the structure is deteriorating to the point where ITD will no longer be able to maintain it for highway traffic. Other issues include narrow lane and shoulder widths, tight curves on both sides of the bridge, and limited sight distance for drivers.

An evaluation in 2025 confirmed the level of corrosive elements in the concrete and rebar is extensive. ITD has recommended removing the bridge due to safety risks, future costs, and a lack of options to keep and use the bridge efficiently. The Federal Highway Administration and ITD will make a final decision later this year once the environmental process is complete. ITD shares regular updates at public meetings.

ITD is working with technical and environmental experts, agencies, and community members to design a new bridge. This process includes: 

• Completing environmental studies regarding wetlands, endangered species, cultural resources, viewshed and socioeconomic resources, the Clean Water Act, and other issues.
• Completing technical studies such as geotechnical and hydraulics work.
• Evaluating a range of alternatives for a new bridge.
• Determining the future of the existing bridge.
• Developing design and construction plans for the bridge and roadway.
• Working with local agencies and the public throughout the process.

This project is funded to start construction in 2028. 

The new bridge will be located slightly northwest and upstream of the existing bridge. This alignment will eliminate sharp curves on both sides of the bridge. In addition:

• State Highway 55 will have two 12-foot lanes and 14-foot shoulders across the bridge.
• Shoulders will be up to 17 feet on the curves before and after the bridge.
• ITD will construct retaining walls near the bridge to stabilize the canyon slopes and give crews room to keep the existing bridge open during construction.

ITD presented nine design options to the community in July and August 2025. The project team is using engineering considerations and detailed public input to recommend a design. An update will be available in late spring 2026.

ITD is approaching decisions about the Rainbow Bridge with a high level of transparency, education and community involvement. The community input process will be a key component of the final recommendation for the new bridge and the existing bridge. 

ITD is hosting a series of community open houses and small-group meetings during this project: 

• Community open houses will be scheduled at key points of the design process. The first set was held in July 2024 in McCall, Horseshoe Bend and online. The second set was held in July and August 2025 in Cascade and online. ITD also hosted a booth at the Valley County Fair.
• A Bridge Involvement Group will meet one week prior to each community open house. The first meeting was held in July 2024 in Cascade. The second meeting was held in July 2025 in McCall Meetings are open to community leaders, stakeholders and any community members who are willing to invest a higher level of time and engagement.

Design is underway. Construction is expected to begin in 2028.