Thousands of separate bridges across the United States are in danger of collapsing if only a single part fails, having been built with zero redundancy.

These risky spans, termed "fracture critical," by engineers and the U.S. government, can collapse if only a single, vital part of the bridge is damaged in any way. This was exactly the case on Thursday afternoon, when a bridge north of Seattle collapsed after an oversized truck clipped a steel truss.

Even bridges that are deemed structurally fine, like the bridge outside Seattle, can fail if they are hit in a critical spot. Several of these spans, which were built without redundancies, date from the fifties and sixties, when the government was trying to complete the nation's interstate highway system cheaply and quickly. About 18,000 "fracture critical" bridges are spread throughout the country.

"They have been left hanging with little maintenance for four decades now," said Barry B. LePatner, who wrote Too Big to Fall: America's Failing Infrastructure and the Way Forward. "There is little political will and less political leadership to commit the tens of billions of dollars needed" to fix them.

Of the $27 billion designated for highway projects under President Obama's stimulus program, only $3 billion of that went to bridge projects, far short of what it would actually cost to repair spans that won't collapse if a single, critical part fails.