6 - Environment

While transportation enhances the quality of our lives, it also generates environmental impacts that can lead to human health problems and ecological damage. Overall, most transportation air emissions in the United States, such as particulates, have declined since 1980 despite significant increases in U.S. population, Gross Domestic Product, and vehicle-miles traveled. Only ammonia among criteria pollutants remains above its 1990 level. However, carbon dioxide emissions from transportation fuel use have risen.