Date: July 20, 1995
Contacts: Barbara J. Rice, Media Relations Manager
Jennifer Cooke, Media Relations Assistant
(202) 334-2138; Internet <>
Publication Announcement

The Clean Air Act Amendments of 1990 raised the importance of air quality as a goal for the transportation sector alongside the more traditional objectives of mobility and safety. Since then, highway projects nationwide have come under intense scrutiny. Once thought to always reduce congestion and air pollution, projects that expand highway capacity are now being questioned. Local jurisdictions that don't meet clean air standards must show that the implementation of proposed transportation plans will not lead to new or greater violations or delay the attainment of clean air goals.

A new National Research Council report reviews the current state of knowledge on how new roads and other highway capacity additions affect traffic flow, travel demand, land use, vehicle emissions, air quality, and energy use. The study committee that prepared the report concluded that the methods required by federal regulation for estimating the effects of proposed traffic improvement projects on air quality are inadequate. They simply do not give policy-makers and planners important 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, the committee said.

"Transportation officials in areas that still don't meet clean air standards must make judgments about the environmental effects of new roads and other improvements based on their reading of the best available information," said committee chair Paul E. Peterson, Henry Lee Shattuck Professor of Government, Harvard University, Cambridge, Mass. "Unfortunately, the government's required method of estimating vehicle emissions calls for a level of accuracy that can't be met by current scientific capabilities."

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 already at the center of legal challenges and threats of litigation in several metropolitan areas, potentially stalling highway construction programs.

Scientific understanding, at this juncture, cannot easily resolve the disputes, the committee concluded. "On the basis of current knowledge, it cannot be said that highway capacity projects are always effective measures for reducing emissions and energy use," said the committee.
"Neither can it be said that they necessarily increase emissions and energy use in all cases and under all conditions." The effects depend greatly on specific circumstances such as the type of highway improvement project, location of the project in the region, extent and duration of pre-existing congestion, prevailing atmospheric and topographic conditions, and the area's development potential.

The committee provided its best judgment of the likely payoffs of pursuing current policies. It suggested that the regulatory focus on curbing increases in motor vehicle travel by limiting highway construction projects is an indirect approach that will probably have relatively small effects on metropolitan air quality by the late 20th and early 21st centuries, when deadlines set by the Environmental Protection Agency (EPA) must be met. Historically, measures to control travel demand have had limited effects.

Moreover, curtailing the expansion of highway capacity has the potential for pitting environmental concerns against economic ones. In the committee's opinion, a more constructive approach to improving air quality would be to look for more technological improvements that can yield greater benefits for air quality compared to the current focus on curbing travel. Improvements such as preheated catalytic converters, engine power enrichment regulations, use of oxygenated or alternative fuels, and more effective vehicle inspection and maintenance programs should reduce emissions further in the near term. In addition, market solutions -- such as the use of time-of-day tolls and other congestion pricing strategies to control travel on new highways -- also have promise, but the feasibility of some of these approaches is untested.

The Clean Air Act Amendments of 1970 mandate air quality standards for six toxic pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulates, and sulfur dioxide. Despite more than 20 years of progress in improving air quality, EPA estimated in 1993 that about 59 million people still lived in areas that violated clean air standards. Deadlines for meeting clean air standards vary with the severity of air quality problems. Approval of federally assisted highway projects can be withheld if a deficiency is not corrected.

With one exception, the committee endorsed all of the report's findings and recommendations. Michael A. Replogle dissented from some of the key findings, concluding that although analysis tools must be improved, they can be adapted to meet current regulatory requirements without substantial delay.

Expanding Metropolitan Highways: Implications for Air Quality and Energy Use is available at or by calling 202-334-3313  or 1-800-624-6242.  Reporters may obtain copies from the Office of News and Public Information at the letterhead address (contacts listed above).

The study was sponsored by the National Research Council's Transportation Research Board, the American Association of State Highway and Transportation Officials, the Federal Highway Administration, and the Environmental Protection Agency. A committee roster is below. The National Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. It is a private, non-profit institution that provides science and technology advice under a congressional charter.

Transportation Research Board


Paul E. Peterson (chair)
Henry Lee Shattuck Professor of Government, and
Director, Center for American Political Studies
Harvard University
Cambridge, Mass.

Paul E. Benson
Supervising Materials and Research Engineer
California Department of Transportation

Robert G. Dulla
Senior Partner
Sierra Research
Sacramento, Calif.

Genevieve Giuliano
Associate Professor of Urban and Regional Planning, and
Director, Lusk Center Research Institute
School of Urban and Regional Planning
University of Southern California
Los Angeles

David L. Greene
Senior Research Staff Member, and
Manager, Energy Policy Research Programs
Center for Transportation Analysis
Oak Ridge National Laboratory
Oak Ridge, Tenn.

Frank S. Koppelman
Professor, Department of Civil Engineering and the Transportation Center
Northwestern University
Evanston, Ill.

Kenneth J. Leonard
Director, Bureau of Strategic Planning
Wisconsin Department of Transportation

Edwin S. Mills
Gary Rosenberg Distinguished Professor of Real Estate and Finance
Kellogg Graduate School of Management
Northwestern University
Evanston, Ill.

Stephen H. Putman
Professor of City and Regional Planning, and
Associate Dean for Graduate Programs
Graduate School of Fine Arts
University of Pennsylvania

William R. Reilly
Catalina Engineering Inc.
Tucson, Ariz.

Michael A. Replogle
Co-Director, Transportation Project
Environmental Defense Fund
Washington, D.C.

Gordon A. Shunk
Research Engineer, and
Manager, Urban Analysis Program
Texas Transportation Institute
Texas A&M University System

Kenneth E. Sulzer
Executive Director
San Diego Association of Governments
San Diego

George V. Wickstrom
Deputy Director and Manager of Technical Services (retired)
Metropolitan Washington Council of Governments
Washington, D.C.

Catherine Witherspoon
Legislative Director
South Coast Air Quality Management District
Diamond Bar, Calif.

Julian Wolpert*
Henry G. Bryant Professor of Geography
Woodrow Wilson School of Public and International Affairs
Princeton University
Princeton, N.J.


Stephen R. Godwin
Director, Studies and Information Services Division

Nan Humphrey
Senior Staff Officer

* Member, National Academy of Sciences