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In the United States, the transportation sector makes up 27 percent of the national carbon dioxide (‘CO2’) emissions. Within the transportation sector, vehicles are responsible for 83 percent of CO2 emissions. While CO2 makes up the largest portion of greenhouse gases (‘GHG’) from vehicles (approximately 95–99 percent), automobile exhaust produces methane and nitrous oxide, and emissions from end of life disposal and air conditioner leaks emit hydrofluorocarbons. Against this context, it is perhaps unsurprising that US regulatory efforts addressing GHG from the transportation sector focus on vehicles. While the US does regulate air pollution emissions from locomotives, marine engines and aircraft, GHG from these sources are not specifically regulated.