In 2006, the Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly known as the Interstate System, turned 50 years old. The 46,747 miles of Interstate highways serve as the backbone of transportation and commerce in the United States. About 67.1 percent of this 2004 mileage was in rural areas, 4.5 percent was in small urban areas, and 28.3 percent was in urbanized areas. In 2004, Americans traveled approximately 267 billion vehicle miles on rural Interstates, 26 billion on small urban Interstates, and 434 billion on urbanized Interstates. Taken together, this represents approximately 24.5 percent of all U.S. travel in 2004.
The Interstate System is growing more crowded; Interstate VMT grew at an average annual rate of 2.8 percent from 1995 to 2004, outpacing the 0.5 percent average annual growth in lane miles over that period. On rural Interstates, 73.7 percent of VMT in 2004 was on pavements with good ride quality; comparable figures for small urban and urbanized Interstates were 65.6 percent and 48.5 percent, respectively. Current spending on rural Interstate highways appears adequate to further improve pavement ride quality and reduce overall highway user costs, if sustained in constant dollar terms. On urban Interstates, significant increases in funding for rehabilitation and expansion would be required to prevent both average physical conditions and operational performance from becoming degraded.
The Interstate System included 55,315 bridges in 2004, 27,648 in rural areas and 27,667 in urban areas. In 2004, about 15.9 percent of rural Interstate bridges were considered to be deficient, including 4.2 percent classified as structurally deficient and 11.7 percent classified as functionally obsolete. Among urban Interstate bridges, about 26.5 percent were considered to be deficient in 2004, including 5.1 percent classified as structurally deficient and 20.5 percent classified as functionally obsolete.