Reducing the total volume of transportation activity is one way to limit GHG emissions from this sector. The most obvious target for such reductions is the transport of passengers and goods on highways, which is responsible for 75 percent of the energy used in transportation. Reducing traffic volume is difficult, however, in light of the interconnections among such factors as choices about where to live and work, the built environment (see Chapter 12), and the availability and flexibility of transportation options.
Improving the fuel economy of highway vehicles and shifting transportation activities away from highways and to modes that have the potential to be more efficient (such as rail and public transit) are also important approaches to reducing emissions. However, whether an alternative mode provides net emissions benefit depends on how it is used. For example, except in a few dense urban corridors, such as in New York City, load factors are not high enough to make public transit less energy and emissions intensive (per passenger-mile) than passenger cars, especially outside of rush hours. The "container revolution"—a shift from truck to rail (and ocean) carriers—has increased efficiency of the transportation of goods. The NRC's Transportation Research Board is currently conducting an in-depth analysis of the technical potential for reducing the energy (and hence emissions) intensity of freight movement.
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