Major changes in U.S. transportation policies, technologies, and practices may become necessary to reduce motor vehicle emissions and the subsequent risk of global warming during the next century. Because transportation plays an integral role in the nation's society and economy, a balance must be struck between the mobility and access needs of people on the one hand, and environmental and natural resource needs on the other.

With the assistance of other units of the National Research Council, TRB formed a committee with expertise in environmental sciences, transportation technologies, industry, public policy, economics, political science, civil engineering, and planning. The project is complete. The TRB committee's final report has been published as Special Report 251, "Toward a Sustainable Future, Addressing Long-Term Effects of Motor Vehicle Transportation on Climate and Ecology."

Cars, buses, and trucks in the United States burn more than 140 billion gallons of gasoline and diesel fuel each year, emitting an average of more than 1 pound of carbon dioxide for each mile they travel. U.S. transportation accounts for about 5 percent of the carbon dioxide emitted as a result of motor vehicle use and other human activities worldwide. Although this percentage may seem modest, no other category of energy use in the United States or elsewhere in the world accounts for a significantly larger portion of global carbon dioxide emissions.

Two general approaches should be considered for reducing motor vehicle emissions of so-called "greenhouse gases" over time. One is to encourage reductions in driving through taxation, pricing, and other policies aimed at changing travel behavior. Another is to encourage the development of new transportation technologies that use either smaller amounts of petroleum fuels or alternative fuels that produce fewer greenhouse gases. These two basic approaches may be complementary. Both would require increased public understanding of global warming risks and how motor vehicles contribute to the buildup of greenhouse gases, as well as greater public support for policies to address this problem.

Policy options either for reducing motor vehicle use or for encouraging new technologies involve many uncertainties about their overall influence on greenhouse gases, their practicality, and their secondary costs and benefits, the report says. More research is needed to develop policies acceptable to the public and to assess how effective they might be, both individually and in combination.

Moreover, vehicle emissions other than carbon dioxide have potentially cumulative and long-lasting effects on the function and biological makeup of Earth's ecosystems. The chemicals emitted in vehicle exhaust are dispersed widely and react in the atmosphere to create compounds that change air, soil, and water chemistry, leading to adverse conditions such as acid rain. While significant progress has been made in understanding the causes of these effects and in reducing high concentrations of air pollution in U.S. metropolitan areas, the longer-term effects of these emissions outside of urban areas have not been extensively monitored or studied.

In addition to emissions, the enduring network of roads, railway track, pipelines, and other transportation facilities can have ecological effects across broad areas, such as altering the flow of water within a regional watershed or impeding the movement of species through their natural ranges. These effects, as well as those from vehicle emissions, may be leading to gradual changes in biological diversity and ecosystem functions on a regional or national scale.

Although the full extent of these environmental changes may not become evident for years, the adverse consequences may be significant and lasting, the report says. Early and deliberate research and policy efforts are needed to meet these unique environmental challenges. Research is needed in areas such as behavioral and social factors that influence transportation demand, to better understand the cumulative effects of motor vehicles and transportation infrastructure on the environment, and to provide guidance to policy-makers. Sustained federal support is needed for research aimed at developing alternative vehicle technologies and fuels, and improving techniques for prevention and mitigation of damage to ecosystems.