A large proportion of GHG emissions can be attributed to transportation, specifically from the burning of gasoline, diesel, and other fuels derived from petroleum. In fact, the transportation sector is responsible for 70 percent of U.S. petroleum use, which exceeds the percentage of oil that is imported (Davis et al., 2008). Reducing transportation’s dependence on petroleum, much of it imported from politically unstable regions of the world, is one of the most direct connections between the issues of climate change, energy security, and national security (see Chapter 16). Transportation’s use of petroleum fuels also leads to emissions of particulate matter, sulfur dioxide (which forms sulfate aerosols and ultimately leads to acid rain), and substances that are precursors to photochemical smog (nitrogen oxides [NO_x] and carbon monoxide [CO]) and to various forms of pollution in freshwater and marine systems. Hence, efforts to reduce GHG emissions in the transportation sector will also confer other benefits to the environment and public health (see Chapter 11).

Transportation activity is typically divided into two categories: the movement of people and the movement of goods. The movement of people, usually expressed in passenger-miles, accounts for 70 percent of the transportation sector’s energy use and GHG emissions (Davis et al., 2008). The principal vehicles involved in the movement of people are light-duty personal vehicles—automobiles and light trucks—and commercial aircraft, which together account of almost 99 percent of passenger-miles (Davis et al., 2008). The movement of goods, usually expressed in ton-miles, is dominated by trucks, railroads, and ships. These freight modes account for the remaining 30 percent of transportation-related emissions (Davis et al., 2008). Table 13.1 shows the relative importance of different modes of personal and goods transport to total transport energy use and, by implication, its approximate contribution to GHG emissions.

In the United States between 1970 and 2007, energy intensity—the amount of energy required to produce a unit of transport activity—declined for nearly all transportation modes (for example, energy intensity declined by 0.3 percent per year on average for medium and heavy freight trucks, 0.8 percent per year for passenger cars, 1.5 percent per year for light trucks, 1.8 percent per year for freight rail, and 3.3 percent per year for domestic passenger air travel). However, these increases in efficiency were more than offset by an increase in total transportation activity (for example, the number of passenger-miles flown grew by 4.9 percent per year), leading to the overall growth in energy use and GHG emissions.