Highway capacity improvements--ranging from better traffic-signal timing to construction of major highways--traditionally have been viewed as ways to reduce pollution and improve fuel use by smoothing traffic flow and raising speeds. But some analysts have argued that these efforts will lead to more traffic, higher emissions, and greater energy consumption by stimulating more motor vehicle travel and auto-oriented land development. The issue is at the center of legal challenges and threats of litigation in several metropolitan areas that must comply with the requirements of the 1990 Clean Air Act Amendments.

The study attempted to review and evaluate the state of knowledge on how new roads and other highway capacity improvements affect traffic flow, travel demand, land use, vehicle emissions, air quality, and energy use. The study committee, with the exception of one member who wrote a dissenting statement, concluded that the methods required by federal regulations for estimating the effects of proposed traffic improvements on air quality do not give policy-makers and planners the information needed to predict reliably the effects of expanding highway capacity. In particular, forecasting models that predict how road changes will affect vehicle emissions should be changed to better reflect typical driving patterns and types of vehicles. However, the committee noted that, even with improved models, the complex and indirect relationship between highway capacity additions, air quality, and energy use--which is heavily dependent on local conditions--makes it impossible to generalize about the effects of added highway capacity on air quality and energy use. Nevertheless, understanding the necessity to comply with regulatory requirements and make decisions on the basis of the best available information, the committee provided its best judgment of the likely payoffs of pursuing current policites. In its opinion, the current regulatory focus on curbing growth in motor vehicle travel by limiting highway capacity is at best an indirect approach for achieving emissions reductions from the transportation sector that is likely to have relatively small effects, positive or negative, on metrolpolitan air quality by current EPA attainment deadlines.

The final report has been published as Special Report 245, Expanding Metropolitan Highways: Implications for Air Quality and Energy Use.