Because there are significant differences in the energy expended per passenger-mile or ton-mile among the major modes of transportation, a second candidate strategy for reducing transportation-related GHG emissions is to shift people or freight to more energy efficient modes. The two most widely discussed options are (1) inducing people to substitute some of their driving with public transportation service, bicycling, and walking; and (2) shifting more freight from truck to rail.
The viability of public transportation (as well as walking and biking) as an alternative to driving hinges in part on there being favorable urban land use patterns, as discussed in the preceding subsection and in the recent report Driving and the Built Environment (NRC, 2009e). For public transportation to be an energy efficient alternative to the private vehicle, however, requires that the services be heavily used. At present, except in a few very dense urban areas such as New York City, public transportation load factors are not high enough to make these services more energy- and GHG-efficient than driving. Because demand is especially low outside of rush hours, transit systems often operate with very low levels of occupancy for much of the day (NRC, 2009c). As a consequence, buses—the most prevalent form of transit—used 24 percent more energy per passenger-mile than private cars in 2006 (Davis et al., 2008). Subways and commuter rail systems, in contrast, used about 20 percent less energy per passenger-mile than private cars, but these systems accounted for a minority of total public transportation ridership.
There is also significant geographic variability in the availability of public transportation: 97 percent of all subway and transit rail trips occurred in metropolitan areas with a population of over 5 million, and the New York metropolitan area alone was responsible for 38 percent of all national transit use for travel to and from work (NRC, 2006a). Bicycling and walking do not emit any GHGs and are associated with health co-benefits, but they currently constitute a very small share of all miles traveled by people when compared with motorized modes. Strategies designed to facilitate and promote these modalities could yield multiple benefits.
There has also been interest in using passenger rail for medium-distance (500 miles or less) intercity travel in the United States, which is currently dominated by automobiles and, to a lesser extent, air travel. In Europe and Japan, high-speed rail is succeeding in winning substantial market share away from automobiles and air transport for city-to-city travel at distances of up to 500 miles (FRA, 2009). There are many challenges, however, to duplicating such a system in the United States. While high gasoline and deisel fuel taxes and road tolls tend to discourage intercity travel by private car in Europe and Japan, the ease and low out-of-pocket cost for automobile travel in the United States favors their use. Automobiles also offer flexibility for local travel once at the final destination, which is particularly important for families and leisure travelers who make trips between suburbs rather than center cities. A large share of business travel also takes place in suburban areas, which are poor locations for high-speed rail terminals. Another challenge is that there are relatively few large U.S. metropolitan areas located within 500 miles of one another, especially when compared with Europe and Japan. Because of the long distances between cities, aviation is the only practical alternative for timely intercity travel in the United States. Moreover, U.S. airlines, operating in vast networks that funnel passengers through hubs, have the passenger volumes required to offer large numbers of flights between city pairs. This ability to offer a dense schedule of flights—which is highly valued by time-sensitive business travelers—cannot be matched by high-speed rail. The recent uptick in intercity bus travel in the United States, which has been attributed both to the recent economic downturn and to higher fuel prices, is another longer-distance travel option that could potentially be promoted to reduce overall energy use and GHG emissions, particularly among leisure travelers.
The practicality and benefits of shifting additional freight traffic from truck to rail has been studied and debated for years. In 1939, 64 percent of freight ton-miles moved by rail, while trucks carried only 9 percent, with most of the remainder moved on waterways (Department of Commerce, 1975). In 2006, rail's share had declined to 40 percent, dominated by heavy, bulk commodities such as coal, while trucking had increased its share to 28 percent (Margreta et al., 2009). Although moving freight by rail is generally more energy efficient than moving freight by truck, it is not clear that a significantly larger share of freight could be practically moved by rail. For example, because many rail sidings have been abandoned, most freight traffic, and especially manufactured goods, are moved by truck for at least a portion of the journey. On the other hand, the containerization of freight—especially for imported goods—increases the potential for movement by rail, and the recent sharp increases in the price of fuel seem to have shifted some containers from truck to rail and some truck trailers to rail (in "piggyback" service) for the line-haul segment of the trip. Observers who study freight movements contend that rail container and trailer movements such as these are generally not economically viable until line-haul distances reach 700 miles and, with the exception of the longest moves (over 1,500 miles), between the most heavily traveled markets having lane traffic densities in excess of 400,000 tons annually (Wittwer, 2006).
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