The future of an iconic Idaho 55 bridge remains in doubt as designs for a replacement bridge have been narrowed by the Idaho Transportation Department.
On Thursday the agency announced three final design options for a new bridge to replace the Rainbow Bridge, which was built in 1933 to span the North Fork Payette River near Smiths Ferry.
Each design option features concrete or steel arches similar to the concrete arch that became the namesake for the current bridge. The designs are now being further studied by ITD, with more public comment opportunities expected next year before a final design is chosen.
“Once we select a bridge type, we can look more closely at aesthetic features such as railing types, pier shapes, and general finish options,” District 3 Engineer Jason Brinkman said in a video posted to YouTube.
Brinkman said the three finalist designs were chosen based on more than 800 public comments the agency received online and during public workshops held over the summer. ITD originally proposed nine possible designs for the replacement bridge.
Construction on the new bridge is currently expected to begin in 2028. Preliminary estimates for the project are about $60 million.
ITD cites space constraints, safety for removal recommendation
ITD is currently recommending removal of the Rainbow Bridge once the replacement bridge is built — a recommendation the agency quietly revealed earlier this year, as Valley Lookout reported.
Obstacles to keeping the bridge in use by pedestrians and bicyclists, or as a historic landmark, include a lack of parking or space to create more parking, Brinkman said.
Even with more parking, Brinkman said the location of the bridge would create a safety hazard for drivers turning onto and off of the highway.
“We simply do not have the room for this,” he said. “We’ve discussed and studied this and we are growingly concerned about this concept.”
Brinkman also said technical studies of the existing bridge indicate “complete replacement of all major components” would be necessary in the future, even for continued use by pedestrians and bicyclists. That cost, according to the agency, could be between about $20 million and $50 million.
The idea of keeping the Rainbow Bridge as a pedestrian bridge was first raised by ITD in a 2021 feasibility study. The concept has since been the subject of “mixed” public opinion, Brinkman said, adding that it was among the most frequently asked questions ITD received about the project.
Without donors like you, this story would not exist.
Make a donation of any size here
“It was clear that people are on board with building a new bridge,” he said. “At the same time many shared how much they appreciate the existing bridge.”
Replacement bridge construction
The replacement bridge would be built north of the Rainbow Bridge, which ITD plans to keep open through construction of the new bridge. The new location of the bridge would soften sharp curves that currently require drivers to slow as they turn onto the Rainbow Bridge, while also widening travel lanes and shoulders.
Retaining walls as high as 40 feet tall would be built along the highway’s approach to the bridge on either side of the river. Brinkman said the walls would feature a natural rock finish, as desired by public comments.
Work completed on the Rainbow Bridge this past summer was needed to extend the life of the bridge through to the completion of the replacement bridge. Repair work, however, was not a long-term solution for the bridge, according to district engineers.
The Rainbow Bridge, an open spandrel concrete arch bridge, was listed on the National Register of Historic Places in 1999. At 410 feet, it is the largest single-span arch in Idaho.
Idaho 55 corridor planning
The bridge replacement is part of a years-long project to revamp the busy state highway through the canyon.
Recent major projects include the addition of a stoplight at the intersection of Idaho 55 and Banks Lowman Road, as well as a $60 million project to straighten and widen the roadway just south of the Rainbow Bridge.
ITD is currently studying Idaho 55 corridor improvements between Smiths Ferry and McCall. A final report is expected to recommend new turn lanes, passing lanes, and other improvements.
One thing the report will not include is a re-routing of Idaho 55 around the narrow, winding river canyon between Round Valley and Smiths Ferry, although the concept was once studied by ITD.
“Re-routing the highway is not an option for us,” Brinkman said. “Any other location would have much more significant environmental impacts and costs.”
