What is the largest source of U.S. transportation emissions?
Road travel is the largest source of U.S. transportation emissions. In 2022, combustion-engine light-duty truck and heavy-duty truck (MHDT) emissions were roughly 645 and 400 MtCO₂e, respectively, while passenger cars generated some 360 MtCO₂e. Passenger cars were historically the highest-emitting vehicles in the U.S., but improved efficiency and stricter emissions standards have slashed GHG emissions from these vehicles by more than 40 percent since 1990. By comparison, MHDT emissions, which are primarily produced by diesel engines, increased by almost 80 percent during the same period.In addition to being major sources of U.S. GHG emissions, road vehicle exhausts are also significant contributors to emissions of harmful air pollutants like nitrogen oxides (NOx). However, unlike with GHG emissions, annual NOx emissions from road vehicles have fallen dramatically over the past decades as a result of stricter vehicle emissions standards.
Decarbonizing the U.S. transportation sector
Curbing emissions from the transportation sector will be vital if the U.S. is to achieve its long-term targets of cutting economy-wide emissions by 50 to 52 percent from 2005 levels by the end of this decade, and a net-zero economy by 2050. This will require a broad range of strategies and policies, behavioral changes, such as taking public transportation instead of driving, improving efficiency in vehicles, as well as increasing the adoption of low-carbon technologies in the automotive industry.In March 2024, in a bid to tackle tailpipe emissions and boost sales of electric vehicles, the Biden administration announced the strongest-ever emissions standards for new light-duty vehicles. Though less strict than previously proposed, the new standards are projected to cap average fleetwide emissions at 85 grams of CO₂ per mile (gCO₂/mi) for model year 2032 — down roughly 50 percent from emission standards for model year 2027. The final rule stands to eliminate more than seven GtCO₂ by the time it is fully implemented in 2055.
