modes of travel, substituting information and communication technologies for travel, enhancing nonmotorized travel, and reorganizing land use to achieve higher population densities.

In order for a diversified and efficient system to evolve, two types of policy changes are needed: better land-use management and greater use of pricing. The former would have to be implemented on a substantial scale to have a significant effect on fuel consumption and greenhouse gas emissions, and the timeframes of such deployments would span several decades, but the long-term implications could be enormous. The net effect of a concerted effort to internalize congestion-related and environmental externalities into prices could also be large, especially in reducing the numbers of single-occupant vehicles on the roads and encouraging the use of small and efficient city cars for local travel.

Numerous factors hinder the improvement of energy efficiency and the reduction of fuel consumption in passenger and freight transportation. Some of the most important are noted in the following list:

• In the United States, many factors—including a century of falling energy prices and rising incomes, together with personal preferences and some government policies—have contributed to decentralized land-use patterns and a transportation-intensive economy.

• Low-priced energy has led to consumer purchasing behavior, vehicle designs, and operating decisions that emphasize convenience, style, and speed over fuel economy in automobiles and light trucks. Changing these preferences, which have been developed and reinforced over decades, will not be easy.

• The primary barriers to realizing greater energy efficiency in the transportation sector are the expectations of individuals and companies about future energy prices, fuel availability, and government policies. Although there is an extensive menu of existing technologies for saving energy in transportation, before decision makers decide to invest in these technologies they must be convinced that energy-price increases (or other factors that influence market demand) will persist.

• Even if sufficient demand exists for certain vehicle technologies, there must be the capacity to supply them at the needed scale—vehicle manu-