Idaho 55 Widening and Rainbow Bridge Replacement

under construction

The Idaho Transportation Department (ITD) is undertaking a project to widen Idaho Highway 55 in Valley County and replace the Rainbow Bridge. The project aims to improve safety on the dangerous two-lane highway that runs through a canyon between Smiths Ferry and Round Valley. The project includes widening the highway, softening sharp curves, and replacing the historic Rainbow Bridge. The first phase began in 2020 and faced challenges such as landslides and slope stabilization, increasing costs from an initial estimate of $18.9 million to over $62 million. The Rainbow Bridge replacement is planned for 2028 with an estimated cost of $60 million. A third phase of improvements on the north side of the bridge is also planned but not yet designed or costed. ITD studied alternatives including rerouting the highway to bypass the canyon but decided against it due to steep slopes, environmental concerns, archeological sites, and higher costs estimated at several hundred million dollars.