On the heels of the sudden closure of a major commuting bridge in Louisville, Ky., a new report shows that more than 18,000 of the nation's busiest bridges, clustered in the nation's metro areas, are rated as "structurally deficient," according to a new report from Transportation for America.

In Los Angeles, for example, an average of 396 drivers cross a deficient bridge every second, the study found. The Fix We're In For: The State of Our Nation's Busiest Bridges, ranks 102 metro areas in three population categories based on the percentage of deficient bridges.

The report found that Pittsburgh had the highest percentage of deficient bridges (30.4 percent) for a metro area with a population of over two million (and overall). Oklahoma City (19.8 percent) topped the chart for metro areas between one to two million, as did Tulsa, Okla. (27.5 percent) for metro areas between 500,000 to one million.

At the other end of the spectrum, the metro areas that had the smallest percentage of deficient bridges are: Orlando, Fla. (0.60 percent) for the largest metro areas; Las Vegas (0.20 percent) for mid-sized metro areas; and Fort Myers, Fla. (0.30 percent) for smaller metro areas.

Nearly 70,000 bridges nationwide are rated "structurally deficient" and are in need of substantial repair or replacement, according to federal data. Metropolitan-area bridges carry 75 percent of the trips that are made on structurally deficient bridges, he noted.

The Federal Highway Administration (FHWA) estimates that the backlog of potentially dangerous bridges would cost $70.9 billion to eliminate, while the federal outlay for bridges amounts to slightly more than $5 billion per year.