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Utah officials say one reason they're looking at a gasoline-tax hike or other transportation taxes is that the state will soon need to ramp up spending to replace or rehabilitate bridges. At the same time, Utah's bridges are rated among the best in the nation.

How to explain the apparent contradiction?

The Utah Department of Transportation says it's simply because many of those bridges are aging. UDOT bridge management engineer Josh Sletten likens them to cars with more than 100,000 miles — they may have been running fine so far, but experience shows that problems and repairs are likely coming.

"We've always had this rule of thumb that a bridge at 50 years old is like a car at 100,000 miles. You still have to evaluate it bridge by bridge, but you can start anticipating that treatments are needed," he said.

UDOT bridge data analyzed by The Salt Lake Tribune show the state owns 1,807 bridges, and 322 are now 50 years old — 18 percent. At the end of the decade, the number of bridges over 50 will rise to 576 — 32 percent. Most of those were built in the 1950s and '60s when freeways here were first constructed.

By 2030, the number of 50-year-or-older bridges will rise to 920 — 51 percent.

Carlos Braceras, UDOT executive director, has told legislators that means while Utah has been replacing about 15 bridges annually in recent years, "we will have to step that up to 50 a year" soon. That will cost an extra $9 million a year, he says, and is among reasons to consider raising gas taxes.

At the same time, Transportation for America ranks Utah bridges fourth best among the states.

UDOT data show that only 23 state-owned bridges — 1.3 percent — are considered "structurally deficient," or need work but are safe enough to remain open. That's a fraction of the 11 percent national average.

"If you go back to 1998, we were at about 10 percent deficient," UDOT's Sletten said. "We've gone from 10 percent to 1 percent" largely because problem bridges were replaced or repaired as part of projects to add capacity to Interstate 15 in Salt Lake County (before the 2002 Olympics) and Utah County (completed last year), and to improve I-80.

Sletten says those replaced bridges were essentially "low-hanging" fruit that were easy to handle through funding for other projects — but few additional such projects are envisioned now. Most of the remaining aging bridges now are in rural sections of I-70, I-15 or I-80 and require specific funding to address, he said.

Sletten also said that bridges tend to deteriorate rapidly when they hit 50, and a third of Utah's spans are approaching that, "You're kind of sitting on the edge of a cliff."

Bridges in areas without much freezing or salt use can last 100 years or more if properly maintained, said Carmen Swanwick, UDOT chief structural engineer. The department tries to stay on top of that by inspecting every bridge at least once every two years. It uses that information to plan when and where to maintain, rehabilitate or replace bridges.

If lawmakers increase revenue for highways, Swanwick expects much of it "will actually go to try to maintain bridges for a longer time."

But Sletten said if officials wait too long for such life-extending work, "the return on investment is very minimal. ... One of our monickers is good roads cost less because if you're doing timely treatments, the benefit of that treatment will give you many more years of service than if you wait."

Utah's 24.5 cents-a-gallon tax on gas and diesel was last increased 16 years years ago. Mayors around the state want an increase of at least 5 cents a gallon.

The Utah Foundation says without such hikes the state would face a shortfall of $11.3 billion for high-priority transportation projects by 2040.