The 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 transportation 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 by car and truck— because vehicles have become more efficient. The use of motor vehicles has expanded with economic growth, but the average fuel economy of new light-duty vehicles (cars and light-duty trucks, including sport utility vehicles, crossover utility vehicles, minivans, and pickup trucks) rose from 20 miles per gallon in 2005 to 25 miles per gallon in 2021.

• Tighter standards for fuel economy and emissions, along with greater use of electric vehicles, are projected to reduce emissions moderately over the next decade. CBO projects CO2 emissions in the transportation sector to decrease by 9 percent from 2021 to 2032 as the vehicle fleet becomes increasingly efficient to comply with more stringent fuel economy standards. Sales of electric vehicles, which accounted for 4 percent of the market in 2021, are expected to grow substantially.

The use of electric vehicles is expected to contribute to greater emissions reductions in future decades than it does today because the electric power sector is projected to continue to become progressively less carbon intensive.