The condition of bridges nationwide has improved markedly since the early 1990s. Of the nearly 600,000 roadway bridges in 2001, the Federal Highway Administration found that 14 percent were structurally deficient and 14 percent were functionally obsolete. About 40 percent of bridges were either structurally deficient or functionally obsolete in 1991 .
Structurally deficient bridges are those that are restricted to light vehicles, require immediate rehabilitation to remain open, or are closed. Functionally obsolete bridges are those with deck geometry (e.g., lane width), load carrying capacity, clearance, or approach roadway alignment that no longer meet the criteria for the system of which the bridge is a part.1 In the 1990s, while the number of structurally deficient bridges steadily declined, the number of functionally obsolete bridges remained fairly constant (figure 85).
In general, bridges in rural areas suffer more from structural deficiencies than functional obsolescence (particularly on local roads), whereas the reverse is true for bridges on roads in urban areas (figure 86 and figure 87) . A large proportion of problem bridges nationwide are those supporting local rural roads: about 71,000 of the 165,000 deficient bridges in 2001 (43 percent) are rural local bridges. Problems are much less prevalent on other parts the highway network. Nevertheless, in 2001, 20 percent of urban Interstate bridges and 12 percent of rural Interstate bridges were deficient.
1. U.S. Department of Transportation, Federal Highway Administration, Office of Engineering, Bridge Division, National Bridge Inventory database, available at http://www.fhwa.dot.gov/bridge/britab.htm/, as of December 2002.
1 Structurally deficient bridges are counted separately from functionally obsolete bridges even though most structurally deficient bridges are, by definition, functionally obsolete.