WASHINGTON — A new study of the condition of bridges across the U.S. finds about 9.5 percent of the nation’s 612,000 bridges were classified as structurally deficient in 2015.

That adds up to 58,495 structurally deficient bridges, which is down 2,574 from the year before. But researchers say it would take at least 21 years to upgrade all of them at the current pace of bridge investment.

“There are some significant challenges out there,” says Alison Black, chief economist with the American Road & Transportation Builders Association, which conducts an annual study of bridge data collected by the U.S. Department of Transportation.

A bridge is labeled structurally deficient when a key element of the structure is in poor or worse condition.

If placed end-to-end, the deck surface of the nation’s structurally deficient bridges would stretch from New York City to Miami, the study finds.

“There are a lot of bridges that really do need some repair,” Black says.

The D.C. area fares a bit better than the nation as a whole. In Virginia, 7.7 percent of bridges are classified as structurally deficient. In Maryland, the figure is 5.8 percent; in the District, it is 3.9 percent.