Nearly 10 percent of bridges in the U.S. are "structurally deficient" and need serious repairs if they are to remain safe, according to a new analysis by the American Road & Transportation Builders Association.

ARTBA's data comes from a U.S. Department of Transportation inventory that ranks bridge safety on a scale of 0 to 9, with 9 being completely safe and those rated 4 and below being "structurally deficient." The label doesn't mean bridges are likely to fall anytime soon. Rather, it means they could become unsafe if repairs are put off.

The good news is there were 2,574 fewer structurally deficient bridges in 2015 compared to the year before, according to ARTBA. The bad news is there are still 58,500 structurally deficient bridges that, if laid end to end, would stretch from New York City to Miami.

The association said it would take 21 years for all the necessary improvements to make the bridges structurally sound, given current levels of government funding for bridge repair.

"It's going to take major new investments by all levels of government to move toward eliminating the huge backlog of bridge work in the United States," said Alison Premo Black, ARTBA's chief economist.

With 5,025 bridges in need of repair, Iowa tops the list of states with the most structurally deficient bridges, ARTBA said. The state is followed by Pennsylvania (4,783), Oklahoma (3,776), Missouri (3,222), Nebraska (2,474) and Kansas (2,303).

At the other end of the spectrum, the The District of Columbia had 10 structurally deficient bridges in 2015, followed by Nevada (35), Delaware (48), Hawaii (60) and Utah (95).

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