KEY: GDP = Gross Domestic Product.
SOURCE: GDP dataU.S. Department of Commerce, Bureau of Econ-omic Analysis, available at http://www.bea.doc.gov/bea/dn/gdplev.xls, as of Apr. 20, 2001. Ton-miles dataU.S. Department of Transportation, Bureau of Transportation Statistics, National Transportation Statistics 2000 (Washington, DC: 2001). Population dataU.S. Department of Commerce, U.S. Census Bureau, available at www.census.gov/population/estimates/nation/popclockest.txt, as of Apr. 20, 2001.