1. Heavenrich and Hellman (1996) Cited in Fig. 4 caption, but not list
2. AEC (1991) America’s Energy Choices: Investing in a Strong Economy and a Clean Environment Report by the Alliance to Save Energy, American Council for an Energy-Efficient Economy, Natural Resources Defense Council, and Union of Concerned Scientists, Cambridge, MA
3. Alson, J, German, J, Gold, K, Larson, R and Wolcott, M (1996) Transportation Energy Demand Models: Why They Underestimate Greenhouse Gas Emissions Paper presented at the Climate Change Analysis Workshop, Springfield, VA, June 6–7, 1996. U.S. Environmental Protection Agency, Washington, DC
4. Aircraft Emissions and the Environment: COx SOx HOx & NOx;Archer,1993
5. Cannon, J S (1995) Harnessing Hydrogen: The Key to Sustainable Transportation Inform, Inc, New York