While the Clean Air Act has controlled pollutant emissions from all air pollution sources, the greatest success can be found in the control of on-road mobile sources. Emissions reductions from motor vehicles have accounted for 84 percent of the total emissions reductions of the six criteria pollutants since 1970.

Air pollutant levels nationally have improved considerably, and although some areas have shown increases, concentration levels in most urban areas, where problems have historically been the most severe, have shown marked improvement in response to stringent controls.

Since 1970, population has increased 38 percent; the number of people employed has increased 68 percent; the Gross Domestic Product has increased 147 percent; the number of drivers has increased 68 percent; and total vehicle miles traveled (VMT) per year have increased 142 percent. Despite these challenges, national on-road motor vehicle emissions have declined 77 percent.

Increasingly tight EPA engine and fuel standards for both cars and trucks have been instrumental in decreasing emissions, and will continue to do so. Emissions reductions have also been the focus of other programs, such as the Congestion Mitigation and Air Quality Improvement Program, which authorized over $8 billion in TEA-21 for transportation projects aimed at reducing emissions.

Transit vehicles account for a very small percentage of total vehicle emissions, less than one percent of the total. Transit operators, however, are still making strides in improving emissions from transit vehicles through the introduction of clean-burning, more fuel-efficient buses. Since 1992, the share of alternative fuel buses in the transit fleet increased from 1.2 percent in 1992 to 7.5 percent in 2000. Alternative fuel transit buses currently operate in 39 States.

Transit use also contributes to the reduction in air emissions from automobile and truck sources. Public transportation produces about 90 percent less volatile organic compounds, more than 95 percent less carbon monoxide, and almost 50 percent less nitrogen oxides and carbon dioxide than private vehicles that transport the same number of people.

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Reducing pollutant emissions from motor vehicles has been the major factor to this trend in cleaner air, while enhancing the community and social benefits of transportation. Technological innovations, cleaner fuels, and targeted highway and transit programs have reduced emissions significantly over the past 30 years, and this trend is projected to continue well into the future.