Since the introduction of motorized transportation systems, economic growth and advancing technology have allowed people and goods to travel farther and faster, steadily increasing the use of energy for transportation. Modern transportation systems are overwhelmingly powered by internal combustion engines fueled by petroleum. Emissions of carbon dioxide (CO₂), the principal greenhouse gas (GHG) produced by the transportation sector, have steadily increased along with travel, energy use, and oil imports. In the absence of any constraint or effective countermeasures, transportation energy use and GHG emissions will continue to increase.

In the U.S. economy, transportation is second only to electricity generation in terms of the volume and rate of growth of GHG emissions. In terms of carbon dioxide, which accounts for 95 percent of transportation's GHG emissions, transportation is the largest and fastest growing end-use sector.¹  Today, the U.S. transportation sector accounts for one-third of all U.S. end-use sector CO₂ emissions, and if projections hold, this share will rise to 36 percent by 2020. U.S. transportation is also a major emitter on a global scale. Each year it produces more CO₂ emissions than any other nation's entire economy, except China. Given its size and rate of growth, any serious GHG mitigation strategy must include the transportation sector.

This report evaluates potential CO₂ emission reductions from transportation in the United States. Measures considered include energy efficiency improvements, low-carbon alternative fuels, increasing the operating efficiency of the transportation system, and reducing travel. Highway vehicles should be the primary focus of policies to control GHG emissions, since they account for 72 percent of total transportation emissions. Passenger cars and light trucks together account for more than half of total sectoral emissions.