Large combination trucks1 represent a small portion of traffic on the U.S. Interstate Highway System . However, because they are heavier and have more axles than other vehicles, they may cause more pavement damage, a measurement that is estimated in terms of vehicle loadings (see box). In urban areas, these trucks made up only 6 percent of traffic volume, but accounted for 77 percent of loadings in 2001 (figure 19). These trucks also make up a greater portion of the vehicles on rural segments of the Interstate Highway System, representing 17 percent of traffic volume and 89 percent of loadings in 2001 (figure 20).
Between 1991 and 2001, large combination truck traffic volume grew from 14 percent to 17 percent on rural roads, while remaining the same on urban Interstate highways . Concurrently, their share of loadings decreased on both rural and urban Interstate highways. Passenger cars, buses, and light trucks, which the Federal Highway Administration aggregates into one category, followed a different trend— representing a declining percentage of traffic volume but a growing percentage of loadings in urban areas .
1. U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 2001, table TC-3, available at http://www.fhwa.dot.gov/policy/ohpi/hss/index.htm, as of Feb. 26, 2003. Source 1. American Association of State Highway and Transportation Officials, Guide for Design of Pavement Structures (Washington, DC: 1993), p. I-10 and appendix D.
1 Large combination trucks weigh more than 12 tons and have 5 or more axles.