effects of additional highway capacity. They concede that adding highway capacity may initially reduce some vehicle emissions and improve fuel efficiency by smoothing traffic flows and reducing stop-and-go traffic, although the benefits may not be as significant as were once believed. However, these positive effects may be eroded over time by growth in travel stimulated by the new capacity. Improved levels of highway service may encourage shifts from less polluting modes of transportation and induce new or longer trips once discouraged by congested conditions. As traffic volume grows, traffic operations may deteriorate, producing levels of congestion comparable with previous conditions but at higher traffic volumes. In the long run, these analysts maintain, new highway capacity will improve access and may encourage development in low-density areas not amenable to transit. Low-density development requires more frequent and longer trips, increasing emission levels and energy use and further degrading air quality.
The issue of highway capacity and air quality is already at the center of legal challenges brought by environmental groups to many additions to highway capacity in metropolitan areas. This issue is likely to receive more attention as metropolitan areas grapple with the stricter requirements of the CAAA. The act allows citizen suits to be brought for the first time against the U.S. Department of Transportation (DOT) and the Environmental Protection Agency (EPA) for noncompliance with legislative requirements and timetables, which opens the door to increased litigation.
Assessment of the precise consequences for air pollution and energy use of any particular addition to highway capacity is uncertain, given the current state of knowledge and modeling practice. Travel demand forecasting models—the basis for determining the effect of increased highway capacity on travel demand—were originally developed to help determine the appropriate size of new capital facilities. They are not well suited to providing the detailed data, such as speed data and travel data by time of day, needed for modeling and analysis of vehicle emissions and air quality impacts. Nor can most current travel demand models adequately measure the effect of improvements in highway service on the amount of travel or on land use patterns, which in turn could affect future demand for travel and its distribution in a region.
Emissions models, which measure the polluting effects of motor vehicle travel, inadequately represent the emission performance of