The Federal Highway Administration oversees the inspection of all of the country's 575,413 bridges, and it classifies almost 20 percent of them as "structurally deficient." A structurally deficient bridge is not necessarily in danger of imminent collapse; for example, the category includes many bridges that are still safe but have vehicle-weight restrictions.
More than 80 percent of all U.S. bridges are located east of the geographic midpoint between the coasts. The skewed distribution is due in part to the way the Midwest and Great Plains states were settled--in the so-called "township-and-range" pattern, which resulted in the laying down of land parcels and roadways in evenly spaced grids; in order to maintain this even spacing, many bridges had to be built over small waterways. Iowa, for example contains more bridges than Washington, Oregon, Idaho, Montana, and Wyoming combined.
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The fact that the nation's midsection has the highest concentration of bridges is closely related to why, as shown on the map above, a large percentage of the region's bridges are structurally deficient. Many of the country's bridges are owned by counties or other small jurisdictions; in the Midwest, where there are large numbers of rural counties with many bridges and few people, bridges have become a serious financial liability. Because the federal government does not normally fund preventive maintenance, these jurisdictions sometimes find it cost-effective to let their bridges deteriorate to levels at which they are eligible for federal repair or replacement funds.
The percentage of structurally deficient bridges should increase in the next ten years, in large part because tens of thousands of bridges built on the interstate highways during the boom years following the Second World War will soon be in need of major repairs. The Secretary of Transportation estimates that federal, state, and local governments will need to increase their yearly funding by almost 40 percent to meet the surging need for bridge rehabilitation.