KEY: GDP = Gross Domestic Product.
SOURCE: GDP data-U.S. Department of Commerce, Bureau of Economic Analysis, available at www.bea.doc.gov/bea/dn/gdplev.htm, as of Apr. 20, 2001. Ton-miles data-U.S. Department of Transportation, Bureau of Transportation Statistics, National Transportation Statistics 2001 (Washington, DC: 2002). Population data-U.S. Department of Commerce, U.S. Census Bureau, available at www.census.gov/population/estimates/nation/popclockest.txt, as of Apr. 20, 2001.