While transportation enhances the quality of our lives, it also generates environmental impacts. Overall, most transportation-related air pollutant emissions have declined since 1980. This follows national mandates for more stringent vehicle emission standards, cleaner burning fuels, and greener automobile technologies. Greenhouse gas emissions from transportation fuel use rose steadily until 2007 before decreasing in 2008 due to the economic recession. However, transportation-related emissions have recently increased.