– The collapse of an interstate highway bridge in Washington state brings new attention to the limits of the country’s infrastructure, especially older structures that were designed with little room for error and were never intended to carry the number of cars and trucks they see today.

The 58-year-old Interstate 5 bridge over the Skagit River in Mount Vernon, Wash., was considered outdated but not structurally deficient. While state police believe an oversize truck hitting the bridge may have contributed to its collapse, an investigation by the National Transportation Safety Board will ultimately pinpoint the cause.

Bridge experts said the I-5 bridge shares a design feature with thousands of other bridges across the country that make them more vulnerable to failure.

The bridge was fracture-critical, which means there is no redundancy in the structure — if one component fails, the whole bridge can collapse. It’s a scenario that’s happened before, and experts say it will happen again if older, obsolete bridges are not replaced or reinforced.

“This is a repetitive story that’s going to play out again like a horrible nightmare,” said Barry Le- Patner, a New York construction lawyer who’s identified nearly 8,000 of the country’s most troublesome spans.

A 40-year-old interstate bridge in Minneapolis that collapsed into the Mississippi River in 2007 also had a fracture-critical design. Thirteen people were killed and scores more were injured. No one was killed or seriously injured in Thursday’s collapse.

“It could have been a lot worse,” said Pat Natale, executive director of the American Society of Civil Engineers.

Another fracture-critical bridge over the Ohio River between Louisville, Ky., and New Albany, Ind., was closed for several months in 2011 after inspectors discovered a large hidden crack that could have proved disastrous. Engineers added reinforcement to the structure for a fraction of the cost of replacement, and Le-Patner said that a bridge like the one on I-5 similarly could have been made stronger.

“Government officials have failed to fund needed maintenance, and these bridges have long passed their intended life span,” he said.

The 607,380 bridges in the United States are an average age of 42 years old, with one in nine rated “structurally deficient,” said the American Society of Civil Engineers, an organization that advocates increased investment.

“There are many places — and probably the one that just collapsed is an example — where bridges are being maintained but not replaced or improved to the extent they need to be because of the lack of available funds,” said Joshua Schank, president and chief executive officer of the Eno Center for Transportation, a research institute based in Washington, D.C.

The oldest parts of the Interstate Highway System are more than 50 years old and need repair and replacement just as states and the federal government confront shrinking funds to build and maintain infrastructure.

Washington Secretary of Transportation Lynn Peterson said that only the section of the bridge that collapsed would be rebuilt. Gov. Jay Inslee said the repair would cost $15 million — a replacement could cost many times more.

Washington state has much incentive to finish the repairs quickly. I-5 is the primary highway corridor along the Pacific Coast, carrying an average of 71,000 vehicles including 10,000 trucks a day, which will have to endure lengthy and costly detours until the bridge is repaired.

U.S. Transportation Secretary Ray LaHood announced that $1 million in emergency federal funds would be available to help repair the bridge. But the bridge will reopen with the same narrow lanes and low clearances it always had — it predates the interstate system and was not constructed to federal standards for interstates. Many bridges on the interstate system, including this one, were “grandfathered in,” said Sean McNally, a spokesman for the American Trucking Associations, an industry group. “This is designed to a different era.”

Natale said it wouldn’t be enough just to patch over the problem and move on. “The problem is when we get into tight financial situations, we cut maintenance and inspections,” Natale said. “When we find a condition like this, we need to act more quickly.”

Bloomberg News contributed to this report.