SOURCES: U.S. Department of Transportation, Bureau of Transportation Statistics, special tabulation, April 2001; based on: total trade, air and water—U.S. Department of Commerce, U.S. Census Bureau, Foreign Trade Division, FT920 U.S. Merchandise Trade (Washington, DC: Various years); all land modes—U.S. Department of Transportation, Bureau of Transportation Statistics, Transborder Surface Freight Data.