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JANUARY | 2021Emissions of Carbon Dioxide in the Transportation SectorDECEMBER | 2022© Tupungato/Shutterstock.comAt a GlanceThe largest source of emissions of carbon dioxide (CO2, the most common greenhouse gas) in the United States is the transportation sector. Emissions from transportation surpassed emissions from the electric power sector five years ago and now constitute two-fifths of domestic emissions from burning fossil fuels. In this report, the Congressional Budget Office provides an overview of CO2 emissions in the trans-portation sector, describing the sources of and trends in such emissions and projecting their future path.• In 2021, CO2 emissions in the transportation sector were 6 percent less than they were in 2005. The decline in emissions from transportation has contributed to a drop of about 20 percent in total CO2 emissions in the United States since 2005; most of that overall reduction has come from the electric power sector. • Reducing emissions from transportation has been difficult because of the value that people place on transportation and the dominance of a single fuel source—petroleum. Demand for transportation is much less sensitive to price changes than is demand for electric power, and people have had few cost-effective alternatives to motor fuels. • Most emissions in the transportation sector come from cars and trucks. Motor vehicles accounted for 83 percent of CO2 emissions from transportation in 2019. Personal vehicles and commercial trucks (the predominant forms of passenger and freight transportation) averaged more CO2 emissions per passenger-mile or ton-mile than most other modes of transportation.• CO2 emissions have declined since 2005—despite an increase in travel ...