(CNN) There are more than 54,000 bridges in the United States that need to be repaired or replaced. That's good news -- relatively speaking -- because a year ago, the number was more than 55,000.

The latest figure comes from a report by the American Road and Transportation Builders Association, a group that advocates for strong investment in transportation infrastructure.

This year's report, looking at data from 2017, found:

The report found that the largest number of defective bridges are in:

Iowa

Pennsylvania

Oklahoma

Missouri

States with the smallest number of structurally deficient bridges are:

Hawaii

Delaware

Nevada

What it means to be structurally deficient

Bridges are regularly rated for safety on a scale of 0 to 9, with 9 meaning "excellent." In order to be considered structurally deficient, one of its major components must measure 4 or below.

Being structurally deficient does not necessarily mean that the bridge is unsafe, but it does mean it's in need of repair.

Repairing the deficiencies is critical. Deteriorating bridges can lead to road closures, which means there's limited access for cargo trucks.

Traffic bottlenecks cost the trucking industry alone $60 billion a year in lost productivity and fuel, said Alison Black, chief economist with American Road and Transportation Builders Association, which "increases the cost of everything we make, buy or export."

The journey to repairing

Although the number of structurally deficient bridges decreased this year, it's only down two-tenths of a percent from the previous year.

That's not a huge improvement.

Because there are so many bridges in the United States, repairing or replacing all of them would take 37 years at the current pace.

"There is improvement," said Black, "but a lot of work to be done."

But because details of his plans aren't finalized yet, it's unclear how much will go toward these deficient bridges.