opposition to toll increases and congestion pricing proposals in the past. These issues are reviewed in the fourth section.

Discussion about congestion pricing in the United States would not have advanced as far as it has without the emphasis on reducing pollution from mobile sources in the Clean Air Act 1990 Amendments (CAAA) and the growing interest in congestion pricing by environmental groups, business interests, and local and political leaders seeking market-based approaches to reducing congestion rather than regulations restricting travel. Plausible estimates of air quality and energy benefits are discussed next, followed by a review of the debate about how congestion pricing might affect the form of urban development and how this, in turn, might affect those firms located in areas or on routes subject to pricing.

Although this chapter reviews available studies, in the absence of any direct experience with congestion pricing in the United States, it is difficult to predict with complete confidence how changes in travel might unfold and how these changes in turn would affect specific groups, air quality, energy consumption, and urban form. There is sufficient experience with traveler responses to toll increases, transit fare changes, and changes in parking pricing to predict the direction and magnitude of changes in total travel within a metropolitan area, but estimating these consequences across different income groups and classes of highway users is more speculative. Available estimates of changes in emissions are also subject to considerable uncertainty. Longer-term consequences for metropolitan development are even more difficult to gauge. It is possible to estimate near-term consequences with models that forecast aggregate changes in travel, but it is important to appreciate the approximate nature of these estimates.

The evidence from past changes in bridge and turnpike tolls, transit fares, and parking fees demonstrates that motorists do respond to changes in price, even for commuting trips (Harvey, Vol. 2). Economists refer to the sensitivity in demand for a good that results from changes in price as the “elasticity” of a response. Elasticities are calculated in percentage terms: a price elasticity of demand of -2.0 would indicate that a 1 percent increase in price would result in a 2 percent decline in demand. When consumer sensitivity to a price increase lies between -1.0 and 0, demand is referred to as “inelastic.” Thus, if the price elasticity is -0.5, a 1 percent increase in price will result in a 0.5 percent decline in demand. In this case, there will