Expect some traffic delays between Boise, McCall as work begins to replace Idaho bridge
Idaho transportation officials are making strides this month toward replacing the state’s most well-known bridge.
The exploratory work, which will impact weekday traffic between Boise and McCall for the month of June, is the first in a line of efforts to build a new permanent span over the North Fork of the Payette River. The multi-year $54 million project will ultimately result in bypassing Valley County’s Rainbow Bridge, which dates to 1933 and is designated in the National Register of Historic Places.
The unique concrete arched structure connecting commuters back and forth from the Treasure Valley to counties north was approved and built during the Great Depression. But the Rainbow Bridge is nearing the end of its lifespan after approaching a century of use, and no longer meets today’s traffic demands and state highway safety standards, according to the Idaho Transportation Department.
Future plans for the Rainbow Bridge are part of the new bridge project, ITD said in a news release. The agency noted its intent to preserve the existing river span in a prior request for proposals.
“ITD has not decided what to do with the existing Rainbow Bridge,” ITD regional spokesperson Sophia Miraglio said in an email to the Idaho Statesman. “The community will have several opportunities to provide input about the new river crossing and the existing bridge. We encourage the public to attend the public meetings this July to share their thoughts and ideas.”
ITD has yet to share the dates and locations of those meetings planned for next month and later this year. Those interested in attending can request to be added to the mailing list by sending an email to info@sh55rainbowbridge.com.
“We intend to approach this project with a high level of transparency, sensitivity and community involvement,” Dan Gorley, ITD’s resident engineer, said in the news release. “Public input will be a key consideration in the final recommendation for the new bridge and the existing bridge.”
Idaho 55 highway corridor especially challenging
ITD will continue its early work on the project this month with test drilling to obtain rock and soil core samples at 18 locations around the existing bridge near Smiths Ferry. Environmental and design work for a new bridge next to the current river span already began in the spring.
The test results from drilling will help with designing the future bridge’s foundation, supports and retaining walls on the existing highway, ITD said. Construction is currently slated for 2027 or 2028.
The mountainous transportation corridor along Idaho 55 is an especially challenging area for building, given seismic activity and the geology of the region, State Geologist Claudio Berti told the Statesman. A half-mile south of the Rainbow Bridge, ITD recently completed a three-year road improvement project on the state highway that included multiple rock slides during construction, which contributed to delays and rising costs.
“The preparation work ahead of the project construction becomes really fundamental to put the best foot forward and so we have the best understanding of the area and foresee as many complications or hazards as we can,” Berti, who also directs the Idaho Geological Survey, said in a phone interview.
Initial ITD estimates place the cost of construction for the bridge replacement at about $54 million, the Statesman previously reported. It cost $74,000 in federal funds to build the Rainbow Bridge in the 1930s, the U.S. Forest Service reported.
Federal dollars are expected to cover about 60% of the overall bridge costs this time around, according to ITD’s most recent seven-year plan.
ITD will start this week with removal of trees and vegetation near the drilling sites so a helicopter can drop off equipment for the drilling operations. Work will take place during the weekdays until Friday at noon, and motorists should expect up to 15-minute delays while work is completed, ITD said.
After test drilling is finished, ITD will re-seed the ground. Updates on the Rainbow Bridge replacement on the project website at: itdprojects.idaho.gov/pages/rainbowbridge
