1. Transportation Energy Data Book, 18th edn., DOE/ORNL-6941, September 1998.
2. EIA Annual Energy Outlook 1999, DOE/EIA-0383 (99), December 1998.
3. Personal correspondence with official of the U.S. Environmental Protection Agency, August 1999.
4. Congressional Research Service Issue Brief, the Committee for the National Institute for the Environment, 89005: Global Climate Change, March 29, 1999.
5. PNGV, Partnership for a New Generation of Vehicles Program Plan, The Government PNGV Secretariat, U.S. Department of Commerce, Washington, DC, November 1995.