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TRB Special Report 245 - Expanding Metropolitan Highways: Implications for Air Quality and Energy Use reviews existing research on the links among highway capacity, traffic flow characteristics, travel demand, land use, vehicle emissions, air quality, and energy use in metropolitan areas; Identifies the conditions most likely to affect emissions and energy use; reviews the reliability of models and analyses that regional and state planning agencies use to forecast travel demand and land use, emission levels, and energy consumption; and recommends research strategies, modeling improvements, and data collection efforts to improve analytic capabilities.

There are few more difficult or vexing local issues than the siting of a new highway or the expansion of an existing one. Local debates revolve around the trade-offs between easing congestion and increasing air pollution, noise, and sprawl. For many years, officials have attempted to respond to traffic congestion by adding highway capacity. Beyond meeting the public’s demand for roads, local and state officials view highway improvements as an integral part of their economic development strategies. Moreover, because vehicles in highly congested traffic operate inefficiently, it is believed that improving traffic flow would reduce emissions. On the other hand, opponents of new capacity contend that it induces more trips; that traffic in free-flow conditions also generates high emissions of some pollutants, particularly ozone precursors; and that the long-term effect of adding capacity is to support sprawling development. Examining these claims and counterclaims with technical evidence requires computer modeling of travel preferences, the resulting vehicle emissions, regional air quality, and the effects of new highway capacity on longterm land development.

For decades, federal funding assistance created an incentive to build new highways. The Clean Air Act Amendments of 1990 represented an attempt to balance this incentive. Once these amendments became effective, metropolitan areas risked losing a share of their federal transportation funding if adding highway capacity would contribute to their failing to meet national air quality standards. Even though the regulations implementing the Clean Air Act Amendments require metropolitan areas to model the consequences of new capacity, the committee that conducted this study concluded that the existing analytical tools are inadequate for the task specified in the regulations. Whether new capacity is better or worse for air quality depends on local conditions. Relatively modest expansions probably have little effect, positive or negative, on air quality or land development in the urban periphery, whereas significant new capacity can, over a period of many years, contribute to increased travel, emissions, and low-density development patterns. Attempting to meet air quality goals through constraints on travel is an indirect policy measure. More direct benefits would accrue from improvements in vehicle technology, effective pricing or taxation of vehicle use, and more effective land development controls, although such measures are not without controversy and involve trade-offs of their own.


Suggested Citation

National Academies of Sciences, Engineering, and Medicine. 1995. Expanding Metropolitan Highways: Implications for Air Quality and Energy Use -- Special Report 245. Washington, DC: The National Academies Press.

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356 pages |  6 x 9 |  DOI:
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Report Contents 1-402

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