There are four possible strategies that could be employed to reduce GHG emissions from the transportation sector:

• Reduce the total volume of transportation activity;

• Shift transportation activity to modes that emit fewer GHGs per passenger-mile or ton-mile;

• Reduce the amount of energy required to produce a unit of transport activity (that is, increase the energy efficiency of each mode); or

• Reduce the GHG emissions associated with the use of each unit of energy.

Each of these strategies is briefly discussed below. Additional details can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c), and the Transportation Research Board report Potential Energy Savings and Greenhouse Gas Reductions from Transportation (NRC, 2010f). The Limiting report concludes that “near-term opportunities exist to reduce GHGs from the transportation sector through increasing vehicle efficiency, supporting shifts to energy efficient modes of passenger and freight transport, and advancing low-GHG fuels.” Achieving large (that is, on the order of 50 to 80 percent) long-term reductions in GHG emissions in the transportation sector, however, would require major technological and behavioral changes (e.g., Fawcett et al., 2009); this in turn implies a need for additional research to support the development and deployment of new and improved transportation modalities.

The most basic—but perhaps most difficult—way to reduce transportation-related GHG emissions is to reduce the total amount of transportation activity. While there has been some attention devoted to reducing total freight transport volumes—by, for example, promoting consumption of locally produced food and goods—most of the attention in this area has focused on reducing personal transportation activity, especially activity by light-duty vehicles. Since 1980, the number of light-duty vehicle passenger-miles has grown at an average rate of 2.3 percent per year (FHA, 2008). This growth has been spurred by, among other factors, the suburbanization of America. As recently as the 1960s, the majority of daily commutes were from downtown to downtown or from close-in suburbs to downtown. Now, the majority of commutes are from suburb to suburb, with the attendant traffic and pollution issues (NRC, 2006a; see also Chapter 12). Suburbanization has also stimulated the increased use of light-duty vehicles for trips other than commuting—for example, according to the National House-