TWIN FALLS — The I.B. Perrine Bridge is not just an icon for BASE jumping. It’s come to symbolize Twin Falls and the Magic Valley, featured on T-shirts, tourist booklets and the council chambers logo.

The bridge creaks as cars and trucks whoosh over the Snake River Canyon. But they’re not any danger to joggers who pass through a tunnel-like section of trail under the bridge. Above on the bridge deck, pedestrians walk the pedestrian pathway across the canyon. And nearby, BASE jumpers pack away their parachutes after leaping from the bridge to a landing spot 486 feet below. The Perrine Bridge is the only structure in North America, jumpers say, where thrill-seekers can hurl themselves 24/7 without permits or regulations.

It was nearing 6 p.m. on a chilly Sept. 21 evening when Troy Gregory jogged down the steps from the pedestrian walkway wearing sweatpants. Gregory, 54, lives nearby and had just completed his daily walk across the bridge.

“It’s just the adrenaline — the big drop off, the vehicles passing right next to you,” he said. “Watching the BASE jumpers is just awesome.”

Last year, Idaho Transportation Department recognized the bridge’s 40th anniversary as a major transportation route and architectural feat in Twin Falls.

How it came to be

The precursor to the I.B. Perrine Bridge, the Twin Falls-Jerome Intercounty Bridge, opened in 1927. It was hailed as the world’s highest bridge and operated as a toll bridge until the state bought it in 1940.

As this bridge fell into disrepair, a weight limit restriction eliminated much of the truck traffic.

The I.B. Perrine Bridge opened to traffic July 31, 1976. The first vehicle to cross for the parade was a Ford Model T roadster pickup previously owned by I.B. Perrine’s sister-in-law. It had also been the first vehicle to cross the Snake River Bridge in 1927.

The bridge’s superstructure is known as a steel deck arch — with a concrete driving deck with arches of weathering steel. The steel is not painted, ITD spokesman Reed Hollinshead noted.

“This type of steel rusts just enough to form a protective patina for the underlying steel,” he said.

While the I.B. Perrine Bridge has been known by that name since it was built, it wasn’t officially named that until 2000, when the Idaho Legislature gave it the official designation.

I.B. (Ira Burton) Perrine is credited as the founder of Magic Valley and Twin Falls. He proposed diverting the Snake River to irrigate the Magic Valley.

Bridge improvements

In 1993, the Idaho Transportation Department added median overhead lighting and replaced expansion joints. The agency repaired joints in 2008 and added utility conduits beneath the deck overhangs. In 2014, the bridge deck received a new polyester concrete overlay, and other miscellaneous repairs took place.

How it’s holding up

The bridge is rated in “good” or “fair” condition, Hollinshead said. The Perrine Bridge was last inspected in September 2016, and it gets inspected every two years.

“The bridge is performing quite well for its 41 years of age,” he said.

But the bridge is starting to get old, District IV Engineer Devin Rigby said. While there are no major deficiencies, ITD will continually watch for them and schedule regular maintenance projects as needed.

Expected lifespan

The Perrine Bridge was built to last 75 to 100 years, but its surface may need to be replaced in another 15 or 20 years, Rigby said. There are no major projects planned for the next five to seven years.

But as 30,000 vehicles per day cross the bridge, it is nearing its capacity for traffic volume — about 33,000 to 36,000 per day. Any more could hinder traffic mobility, ITD staff said.

The I.B. Perrine Bridge has been projected to see traffic volumes of 42,770 vehicles per day by 2030.