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 hold Travel Survey, in 2001 commuting accounted for 27 percent of all vehicle trips per household while "household-serving" travel (e.g., shopping errands, chauffeuring family members) accounted for most of the remainder (BTS, 2001).
Both logic and empirical evidence suggest that developing at higher population and employment densities results in trip origins and destinations that are closer to one another, on average, leading to shorter trips on average and less vehicle travel. Shorter trips can also reduce vehicle travel by making walking and bicycling more viable as alternatives to driving, while higher densities make it easier to support public transit. A recent National Research Council report, Driving and the Built Environment (NRC, 2009e), examined the relationships between land use patterns and vehicle miles traveled and concluded "[l]ooking forward to 2030 and, with less certainty, to 2050, it appears that housing preferences and travel patterns may change in ways that support higher-density development and reduced [vehicle miles traveled], although it is unclear by how much." While the study concluded that significant increases in more compact, mixed-use development result in only modest short-term reductions in energy consumption and CO2 emissions, these reductions will grow over time. The implications of this and other findings for limiting GHG emissions from the transportation sector can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c).
Another trend that has led to increased travel activity has been the reduction over time in the average number of people traveling in each automobile and light truck. In 1977, the average vehicle carried 1.9 people; by 2001, this had declined by 14 percent, to 1.6 people. For travel to and from work, the average declined from 1.3 to 1.1 (Hu and Reuscher, 2005). Increasing the average vehicle occupancy could lead to reductions in total vehicle miles traveled and thus GHG emissions, even considering small offsets due to the need to pick up and drop off the additional passengers. Many municipalities have instituted policies to encourage carpooling; however, few of these policies were developed based on research on patterns and determinants of human behavior or effective mechanisms for informing such behavior, and there is a need for more evaluation of effectiveness.
Because commuting only accounts for about a quarter of passenger trips, carpooling strategies have limited potential for reducing transportation-related GHG emissions. However, it may be possible to increase the prevalence of ridesharing through more effective conveyance of information and the provision of incentives, both in monetary and convenience terms. New technologies could help in this regard; for instance, it is already possible to use personal telecommunications devices and computers to connect drivers with prospective riders to create casual forms of carpooling. Such op portunities will increase. Indeed, it is conceivable that in some locations public transit services will evolve away from the large fixed-route systems into smaller van-type vehicles that employ dynamic routing technologies to offer transportation services similar to that of private cars but with higher average occupancy (WBCSD, 2004). While such concepts are in limited use in Europe, they have not been explored in the United States.
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