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 (81 percent in 1997) are flammable and combustible liquids, primarily petroleum products. There are an estimated 800,000 hazardous materials shipments per day (or 1.5 billion tons per year).
U.S. Department of Transportation, Research and Special Programs Administration, Office of Hazardous Materials Safety, Hazardous Materials Shipments (Washington, DC: October 1998).
1 The nine classes are: explosives; flammable, nonflammable, and poisonous gases; flammable and combustible liquid; flammable, spontaneously combustible, and dangerous-when-wet materials; oxidizers and organic peroxides; poisonous materials and infectious substances; radioactive materials; corrosive materials; and miscellaneous hazardous materials.