• Improve vehicle efficiency. Major gains in fuel efficiency are technically feasible for cars, trucks, and airplanes. There is evidence, however, that consumers undervalue fuel savings when purchasing new vehicles. In addition, the environmental and security benefits of fuel efficiency are external—i.e., dispersed throughout society rather than to the individual consumer. Because fuel efficiency is thus undervalued in the market place, policies are essential to pull efficiency improvements into the market. The current system for setting vehicle efficiency standards could be made more effective by providing longer lead times for tougher standards. Another option would be to require light trucks to meet standards as stringent as those for cars. Because it takes time for the vehicle fleet to turn over, programs must be initiated now and sustained over decades to realize this technological potential.
• Substitute low-carbon fuels for carbon-intensive fuels. Many alternative fuels produce less carbon dioxide (CO₂) per unit of energy than petroleum. Petroleum, however, has many advantages and is supported by an extensive and well-functioning infrastructure, so policy intervention would be required to spur a transition to alternative fuels. Requiring the use of ethanol as a gasoline additive could yield a 3 percent net reduction in GHG emissions in the near term and a 10 percent reduction in the long term, while maintaining the current fueling system.¹ Work should also start now to lay the groundwork for longer-term solutions, such as a hydrogen-based transportation system.
• Increase transportation system efficiency. Numerous transportation modes—such as air, water, rail, car, bus, and bicycling—exist to move people and goods. Increasing the efficiency of the transportation system would require both improving accessibility to the various modes of transportation and using more efficient ones. Which mode is most efficient depends on the distance traveled as well as population density. In the United States, the evolution over decades of automobile dependence and land use patterns has resulted in an energy-intensive transportation system. Policy options for increasing system efficiency include funding public transportation, building infrastructure that eases the transfer of freight and passengers between modes, supporting “intelligent transportation” technologies. and promoting “smart growth.”

Greenhouse gas emissions consequences are now unaccounted for in public as well as private transportation decisions. Taking climate change into account in these decisions would provide a major impetus to improve vehicle efficiency, substitute low-carbon fuels, and increase transportation system efficiency. Policy options include building institutional capacity at all levels of government to address the climate consequences of transportation, incorporating climate change as a consideration in disbursing monies from the federal Highway Trust Fund, and developing a greenhouse gas cap and trade program to constrain emissions at the lowest possible cost.

No single policy approach will be sufficient. Reducing GHG emissions from transportation calls for a balanced combination of cost-effective measures. Many of the policy measures discussed in this brief do much more than reduce CO₂ emissions. For example, since U.S. transportation is almost entirely fueled by petroleum, decreasing GHG emissions from this sector would also decrease dependence on imported oil.