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Efforts to improve urban air quality have shown positive results in some regions, however, in several major urban areas air quality has not significantly improved despite drastic reductions in allowed emissions from individual vehicle sources. The primary reason for this is the dramatic growth in both the number of vehicles and, due to relatively low energy prices, the number of miles driven. In addition, pollution also occurs in connection with the operation of the large production and distribution infrastructure for gasoline and diesel fuels. As a result, extensive non-attainment areas continue to exist. Further mitigation of vehicle and vehicle-related emissions have been enacted as part of the Clean Air Act Amendments of 1990, but it is generally recognized that further air quality improvements in this area are becoming increasingly costly.