One in nine US bridges - more than 66,000 in total - is structurally deficient, a report has found.
Since 2008, the pace at which the US repairs bridges has slowed threefold compared to 1992-96, the report by Transportation for America, a coalition of transport interests, found.
The average age of US bridges is now 43 years, seven years short of the average lifespan, the report found.
It warns the backlog of bridges needing repair will grow amid US budget woes.
The report recommends US policymakers enshrine bridge repair and overhaul as a top priority for federal transport funds, but notes repairing America's deficient bridges will cost $76bn (£49bn).
Not necessarily unsafe
In 2012, 11% of US bridges were rated structurally deficient, down from 22% in 1992. But the rate of repair has significantly slowed despite an influx of funding from President Barack Obama's 2009 economic stimulus package.
In 15 of the 50 US states, the number of deficient bridges has increased since 2011.
In 10 years, one in four bridges will be older than 65 years.
According to the report, about 260 million trips are taken across deficient bridges each day.
While structurally deficient bridges are not necessarily unsafe, they are generally rated in poor condition and require upgrading or replacing.
"Bridges may be rated deficient for a range of reasons and not all of them pose an immediate threat to public safety," the authors wrote.
"However, allowing bridges to remain in serious need of repair can lead to the sudden closure of a critical transportation link or, far worse, a collapse that results in lives lost and a major economic impact to the affected region."
In 2007 for instance, 13 people were killed when a bridge collapsed in Minnesota, while eight died in Texas in 2001. A bridge collapsed in May in Washington state, but no-one was killed.
The National Transportation Safety Board eventually determined that the 2007 collapse was related to a design error, although the bridge was deemed structurally deficient at the time.
And the bridge over the Skagit River in Washington collapsed in May when a lorry hit an overhead support beam.
The bridge was rated "functionally obsolete", and Transportation for America report noted it had been built before the interstate system and was not designed "to carry the large load of today's interstates".
The American Society of Engineers, meanwhile, believes politicians are too keen to build new bridges instead of repairing old ones.
A new freeway bridge being built in Arizona collapsed on Wednesday, killing one worker and injuring another.