Transportation and mobility are central to the American economy and way of life. While the world's desire for personal mobility and commercial transportation has evolved, so have the geopolitical and environmental landscapes. Global demand for oil is at an all-time high, driving up the price of gasoline and diesel fuel, and the environmental consequences of mobile source emissions have become more and more apparent. More than 100 million Americans live in counties that do not attain federal clean air standards for ozone (O3), sulfur dioxide (SO2), particulate matter (PM), or other air pollutants, mostly because of emissions from the transportation sector. Transportation sources are also responsible for emitting prodigious amounts of carbon dioxide (CO2), a greenhouse gas that is accelerating global climate change. In fact, transportation was the second largest source of CO2 in the United States in 2004, accounting for approximately 30 percent of national CO2 emissions. In response, the U.S. Environmental Protection Agency's (EPA's) Office of Transportation and Air Quality (OTAQ) is spearheading a range of programs to reduce mobile source emissions.