The U.S. Department of Transportation's Hazardous Materials Information System (HMIS) is the primary source of national data on hazardous materials transportation safety. Hazardous materials, as defined in regulations, include nine classes of gases and liquids and other substances.1 However, the vast majority of the hazardous materials shipped within the United States each year (82 percent in 2002) are flammable and combustible liquids, primarily petroleum products. Incident reporting requirements were extended to intrastate motor carriers on Oct. 1, 1998, which may partly explain the subsequent increased volume of reports. Beginning in April 1993, there was a sharp improvement in reporting of incidents by small package carriers.
U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics and U.S. Department of Commerce, U.S. Census Bureau, 2002 Commodity Flow Survey, Hazardous Materials (Washington, DC: December 2004), table 2a.
1 The nine classes are: explosives; gases; flammable liquid; flammable solid; oxidizer, organic; poison (toxic); radioactive; corrosive; and miscellaneous hazardous material. Some classes are further divided into subclasses.