SOURCES: 1994—U.S. Department of Transportation
(USDOT), Pipeline and Hazardous Materials Safety Administration (PHMSA),
Hazardous Materials Information System database, available at
http://hazmat.dot.gov/pubs/, as of April 2005. 1995-2004—USDOT, PHMSA, Hazardous Materials Information System,
10-Year Hazardous Materials Data, available at http://hazmat.dot.gov/pubs/,
as of May 2005.