Pipelines showed the largest increase in system mileage from 2005 to 2006 of any components of the transportation system. Hazardous liquid pipeline mileage increased 6.2 percent, and gas pipeline mileage increased 6.7 percent.

The number of U.S. airports increased by 358, rising from 19,983 in 2006 to 20,341 in 2007. However, the number of certificated airports1 declined from 604 in 2006 to 565 in 2007 (Table 2-1-5: Number of U.S. Airports).

The value of the nation’s transportation capital stocks (the value of infrastructure and conveyances) increased 6.8 percent from 2006 to 2007, to $6.3 trillion (Table 2-1-4: Transportation Capital Stock by Selected Modes). The largest increase was in the value of public highways and streets, increasing $277 billion to $2.6 trillion in total value.

The number of structurally deficient and functionally obsolete bridges in the United States declined from 2006 to 2007, with 1,624 fewer structurally deficient bridges and 513 fewer functionally obsolete bridges (Table 2-1-9 Condition of U.S. Highway Bridges).

Class 1 railroads increased the number of crossties replaced by 9.7 percent, from 13.4 million in 2005 to 14.7 million in 2006 (Table 2-1-13: Crossties Replaced or Added by U.S. Class 1 Rail roads). Railroads also increased the tons of rails replaced by 38,272 (9 percent) (Table 2-1-12: Rail Replaced or Added by U.S. Class 1 Railroads). In addition, railroads placed 1,080 new or rebuilt locomotives into service, representing 4.6 percent of the fleet, as well as 76,528 new or rebuilt freight cars, representing 5.7 percent of the fleet (Table 2-1-14: New and Rebuilt Locomo tive and Freight Cars).

1 Airports serving any: (1) scheduled passenger-carrying operations of an air carrier operating aircraft designed for more than 9 passenger seats; and (2) unscheduled passenger-carrying operations of an air carrier operating aircraft designed for at least 31 passenger seats.