More than one-quarter of total U.S. greenhouse gas emissions come from the transportation sector (see Figure 1), making transportation the second largest source of greenhouse gas emissions in the United States after the electric power sector.
Figure 1: U.S. Greenhouse Gas Emissions by Sector (2012)
Source: U.S. Environmental Protection Agency (EPA), Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012, Table ES-7, 2014. http://www.epa.gov/climatechange/emissions/usinventoryreport.html.
The transportation sector consists of passenger vehicles (a category including both passenger cars and light-duty trucks), medium- and heavy-duty trucks, buses, and rail, marine, and air transport. Of the various transportation modes, passenger vehicles consume the most energy (see Figure 2). Greenhouse gas emissions mirror energy use by each mode, because all modes use petroleum fuels with similar carbon contents and thus greenhouse gas emissions.
Figure 2: Transportation Energy Use by Mode (2012).
Source: U.S. Department of Energy. Transportation Energy Data Book, Table 2.5, 2014. http://cta.ornl.gov/data/chapter2.shtml
The vast majority of transportation emissions (95 percent) are composed of carbon dioxide (CO2), which is released during fossil fuel combustion. An additional one percent of total transportation GHG emissions come from methane (CH4) and nitrous oxides (N2O), emissions also associated with fossil fuel combustion. The leakage of hydrofluorocarbons (HFCs) from vehicle air conditioning systems is responsible for the remaining four percent of transportation GHG emissions. Transportation sources also emit hydrocarbons (which are ozone precursors), carbon monoxide (CO), and aerosols. These substances are not counted as greenhouse gases in transportation emissions inventories but are believed to have an indirect effect on global warming, although their impact has not been quantified with certainty.
Factors Affecting Transportation Emissions
Transportation energy use and emissions are determined by four interrelated but distinct factors: the type of fuels or energy sources, the vehicles, the distance traveled, and the overall system infrastructure.
- Fuel Types and Energy Sources
The transportation sector is the largest consumer of petroleum-based fuels in the United States. Importantly, transportation accounts for about 70 percent of U.S. oil consumption, which greatly affects U.S. energy security.
Figure 3: Petroleum and Other Liquids Production and Consumption, 1970–2013.
Source: U.S. Energy Information Agency (EIA), Monthly Energy Review 2014, Table 3.1, 3.7c, 2014. http://www.eia.gov/totalenergy/data/monthly/index.cfm#petroleum
Nearly all fossil fuel energy consumption in the transportation sector is from petroleum-based fuels (92 percent), with a small amount from renewable sources (5 percent) and natural gas (3 percent). There are several types of petroleum fuels used for transportation. Table 1 lists the major petroleum-based transportation fuels and the volume consumed in the United States in 2013.
Table 1: Estimated U.S. Transportation Sector Petroleum Consumption (2013), Million Gallons.
Distillate Fuel Oil (Diesel)
Residual Fuel Oil
Liquefied Petroleum Gases
Source: U.S. Energy Information Administration (EIA), Monthly Energy Review, Table 3.7c, 2014. http://www.eia.gov/totalenergy/data/monthly/index.cfm#petroleum
Petroleum fuels are supported by an extensive and well-functioning infrastructure and have the benefit of high energy density, low cost, and a demonstrated ability to adapt to a range of operating conditions.
The production and consumption of biofuels has increased significantly since 2005, due to the state and federal renewable fuel standards, which mandate minimum annual consumption levels of ethanol and biodiesel, the two renewable biofuels. Ethanol is an alcohol produced from crops such as corn, vegetable waste, wheat, and others; it is usually combined with gasoline to increase octane levels and more efficient fuel utilization. Biodiesel is produced from natural oils like soybean oil and functions only in diesel engines. In 2013, ethanol (4 percent) and biodiesel (0.7 percent) made up nearly five percent of the total primary energy consumed in the transportation sector.
Over the last 35 years, the fuel economy (miles per gallon, mpg) of new passenger vehicles in the United States has improved significantly, increasing by more than 50 percent. Until very recently, most of the gains occurred in the early years of fuel economy regulation under the Corporate Average Fuel Economy (CAFE) program. Fuel economy improvements were nearly stagnant from the late 1980s to the early 2000s. Over this period, the technical efficiency (amount of energy needed to move a given vehicle mass) of light-duty vehicles improved, although fuel economy (the amount of gasoline consumed per mile traveled) remained unchanged, as consumer preferences shifted to larger, heavier, and more powerful vehicles. Fuel economy standard for light trucks were increased slightly in 2003, and recent federal vehicle standards adopted in 2010 and 2012 are expected to raise average fuel economy as high as 54.5 mpg for model year 2025.
Transportation modes other than passenger vehicles also have efficiency improvement opportunities. For instance, aircraft energy intensity has historically improved at an average rate of 1.2-2.2 percent per year, although aircraft energy intensity steadily plateaued through the 1990s and early 2000s due to both historically low fuel prices and a tripling in the average age of aircraft and engine production lines since 1989. In addition, federal vehicle standards for medium- and heavy-duty vehicles were adopted in 2011, and should improve fuel efficiency significantly.
Figure 4: Corporate Average Fuel Economy (CAFE) Standards vs. Sales-Weighted Fuel Economy Estimates.
Source: NHTSA, Summary of Fuel Economy Performance, 2014.http://www.nhtsa.gov/fuel-economy
- Vehicle Use and Distance Traveled
The third factor that affects transportation emissions is the amount of vehicle use and distance traveled. Transportation demand is influenced by the geographic distribution of people and places, especially the density of development and zoning. Over the past 50 years, on-road vehicle miles traveled (VMT) steadily increased until 2008, and has since declined slightly because of high fuel costs and slowing economic growth.
The absolute growth in distance traveled for modes has been similar. The use of all transportation modes (particularly freight transport and air travel) is still projected to grow rapidly in the future.
Figure 5: Annual On-Road Vehicle Miles Traveled (VMT).
Source: U.S. Department of Energy, Transportation Energy Data Book, Table 3.7, 2014. http://www-cta.ornl.gov/data/chapter3.shtml
The overall operation of the transportation system also plays an important role in GHG emissions. For example, congestion results when transportation demand exceeds capacity and poses a challenge for almost all modes of transportation, from on-road and highway transport, air, and rail. Shifting travel to other modes can reduce congestion, as can electronic signaling and other measures to smooth traffic flows. Reducing congestion has the benefit of lowering fuel consumption and GHG emissions by decreasing the time spent idling. For freight (via rail, truck, and ship) and air traffic, system improvements that allow vehicles to take more direct routes from origin to destination can reduce energy use and emissions.
Transportation activity is expected to grow significantly in all countries of the next 25 years. Over the next two decades, vehicle ownership is expected to double worldwide, with most of the increase occurring in non-OECD countries. The U.S. Energy Information Administration projects that non-OECD transportation energy use will increase by an average of 2.8 percent per year from 2010 to 2040, compared to an average decrease of 0.3 percent per year for OECD countries. Figure 6 shows projected worldwide energy consumption in the transportation sector.
Figure 6: Global Projections for Transportation Sector, Liquids Consumption, 2010-2040.
Source: U.S. Energy Information Agency, International Energy Outlook 2014. http://www.eia.gov/forecasts/ieo/more_overview.cfm
Transportation Sector GHG Mitigation Opportunities
Reducing GHG emissions from transportation will require a systematic approach to address the four interdependent yet distinct components of the sector.
- On the fuels side, transitioning to low-carbon energy sources, such as advanced biofuels or electricity produced from renewable sources, can directly reduce the carbon emissions from fuel consumption.
- Significantly more efficient transportation equipment is needed to complement the transition to low-carbon fuel sources. Alternative vehicle designs include flexible fuel vehicles that can run on a mix of biofuels and petroleum-based fuels or are powered by electricity and stored on-board in batteries or by hydrogen fuel cells.
- Vehicle travel demand is affected by a number of factors. Changing land use patterns and increasing alternative travel options, such as biking, walking or rail, can reduce the use of more energy-intensive modes of transportation.
- Increasing the efficiency of the transportation system would require both improving accessibility to and performance of the various modes of transportation and using more efficient ones. Advanced traffic monitoring and signaling can reduce congestion and improve the overall efficiency of the transportation system.
A strategy to reduce GHG emissions from the transportation sector will need to take into account the potential efficiency improvements for each mode of transportation and determine the appropriate reduction strategy for each. Policies that facilitate the adoption of low-carbon technologies and align infrastructure development and land use planning with GHG reduction goals can lead to further GHG reductions in these areas.
Several studies have analyzed the most cost-effective approach to emission reductions in transportation. Some of these studies include:
C2ES Work on Transportation
Achieving emission reductions and oil savings from the transportation sector requires a multi-pronged approach that includes improving vehicle efficiency, lowering the carbon content of fuels, reducing vehicle miles traveled, and improving the efficiency of the overall transportation system.
At C2ES, we focus on all aspects of transportation from improving vehicle technology to the benefits of land-use planning. We produce cutting-edge research; track policy progress at the state, federal, and international level; blog on current transportation issues; and create and maintain an online resource of transportation technology.
Cutting-Edge Research - In our 2011 report titled Reducing Greenhouse Emissions from U.S. Transportation, we identify cost-effective solutions that will significantly reduce transportation's impact on our climate while improving our energy security. See all our transportation-related publications.
Convening Stakeholders - We've run a multi-year stakeholder dialogue to help enable a national electric vehicle market in the United States. The PEV Dialogue Initiative is a one-of-a-kind effort aimed at identifying policies and actions by public and private stakeholders to accelerate the deployment of electric vehicles.
Policy Progress - We track action at the state, federal, and international level. Our state maps provide useful overviews of action to promote alternative technologies. We also summarize action in Congress and in the Executive Branch, such as our summaries of the Renewable Fuel Standard and Vehicle Fuel Economy and Emission Standards. Lastly we track action at the international level, such as our comparison of international fuel economy standards.
Climate Compass Blog - On our blog, we provide C2ES's take on the latest news from the transportation sector. See our transportation blog postings.
Climate TechBook - The transportation section of the Climate TechBook introduces different modes of transportation along with policies to help mitigate GHG emissions and save oil. Below is a list of all the transportation-related factsheets within the TechBook.
U.S. Department of Transportation (DOT)
U.S. Department of Energy (DOE)
U.S. Environmental Protection Agency (EPA)
Joint Federal Programs
Transportation Documents by the Natural Resources Defense Council
The World Resources Institute Center for Sustainable Transport: EMBARQ
AASHTO Transportation and Climate Change Resource Center
Related Business Environmental Leadership Council (BELC) Companies
 Source: U.S. Environmental Protection Agency (EPA), Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010, 2012. http://www.epa.gov/climatechange/emissions/usinventoryreport.html