Transportation Modes

Quick Facts

  • Transportation activity and vehicle ownership is expected to grow significantly worldwide over the next several decades. The transportation sector, however, offers some of the greatest potential to alter the growth path in energy consumption, as illustrated by the expected effects of increased fuel economy and greenhouse gas standards for vehicles on overall energy consumption in the United States.
  • In the United States, passenger or light-duty vehicles are the largest source of energy consumption and greenhouse gas emissions within the transportation sector. Medium- or heavy-duty vehicles make up many commercial vehicle fleets; these fleets consume large quantities of fuel because of intensive use and the low fuel economy of their vehicles.
  • Aircraft emissions in the United States are a small percentage of total transportation sector emissions, but are expected to grow significantly over the long term. Emissions from marine transportation are a very small percentage of current transportation sector emissions in the United States, with little growth expected over the next 30 years.

Background

The transportation sector consists of cars and light-duty trucks (also referred to as passenger vehicles), medium- and heavy-duty trucks, buses, trains, ships, and aircraft. Energy use and, as a result, greenhouse gas emissions from each mode are determined by four major elements: the fuels used and their carbon content, the efficiency of each vehicle, the distance traveled, and the overall efficiency in transportation system operations. (See Transportation Overview)

Of the various transportation modes, passenger vehicles consume the most energy (see Figure 1). Despite major efforts to shift away from petroleum, oil accounted for over 95 percent of the energy used in transportation in 2011, although biofuels accounted for almost 10 percent of energy used for light-duty vehicles.[1]

Figure 1: Transportation Energy Use by Mode (2011)

* Commercial light trucks are medium-duty trucks weighing between 8,500 and 10,000 pounds.

Source: U.S. EIA. 2013. "Annual Energy Outlook 2013." U.S. Energy Information Administration. April. Accessed August 15, 2013. http://www.eia.gov/forecasts/aeo.

Over the next 30 years, analysts expect energy use for rail, aircraft, buses, and freight trucks to grow at higher average annual rates than energy use for light-duty vehicles, which is expected to decline due to federal fuel economy and greenhouse gas standards; see Figure 2.

Figure 2: Average Annual Growth in Transportation Energy Use by Mode (2011-2040)

Overall transportation energy use is expected not to change from 2011 to 2040. Light-duty vehicle energy use is expected to decrease due to the recent fuel economy and greenhouse gas standards.

Source: U.S. EIA. 2013. "Annual Energy Outlook 2013." U.S. Energy Information Administration. April. Accessed August 15, 2013. http://www.eia.gov/forecasts/aeo.

This factsheet gives a brief overview of the various transportation modes and discusses efficiency improvements available for each.

Passenger Vehicles

Light-duty vehicles, or passenger vehicles, are defined as cars or light-duty trucks with a gross vehicle weight of less than 8,500 pounds. They are the largest source of energy consumption and greenhouse gas emissions within the transportation sector.

Figure 3: Passenger Vehicle Statistics in the United States (1980-2011).

Annual statistics for cars and light trucks: vehicle sales and registrations, vehicle miles traveled (VMT), average new vehicle fuel economy, and federal fuel economy standards.

Source: ORNL. 2013. "Transportation Energy Data Book Edition 32." Oak Ridge National Laboratory. Accessed August 14, 2013. http://cta.ornl.gov/data/download32.shtml.  

Technology options to reduce fuel consumption and greenhouse gas emissions from passenger vehicles can include the following:

  • Technology improvements for conventional vehicles: The technological improvements for passenger vehicles can be grouped according to application: engine efficiency, transmission, and other improvements such as vehicle weight reduction, aerodynamic improvements, and reduced rolling resistance. One significant engine efficiency improvement, the hybrid electric vehicle, has been on the road for over a decade. There is a range of hybrids available today, and they are expected to make up about 10 percent of annual passenger vehicle sales by 2040.[2] In 2011, hybrids made up about 3 percent of the U.S. passenger vehicle market.[3]
  • Plug-in technology: All-electric vehicle, plug-in hybrid, and extended range electric vehicle technologies can eliminate or significantly reduce gasoline consumption. All-electric vehicles are very efficient vehicles that can only be powered by batteries. Extended range electric vehicles offer considerable improvements in fuel economy over conventional hybrids because a battery-powered electric motor can run the vehicle on its own, which is more energy efficient than an internal combustion engine or hybrid vehicle drivetrain. Once the battery is depleted, the vehicle’s internal combustion engine can power the vehicle, giving it a range comparable to that of a conventional vehicle. A plug-in hybrid operates like a conventional hybrid, but with a larger battery pack that is capable of powering the vehicle on its own. Key hurdles for electric vehicles include the development of batteries with higher capacity and longer durability, reducing upfront cost, and deploying needed charging infrastructure. Many electric vehicle models are now available and over 100,000 were sold in the United States between 2011 and mid-2013.[4] The EIA expects electric vehicles to make up 3.8 percent of the passenger vehicle market in 2040.[5]
  • Hydrogen fuel cells: Hydrogen fuel cell vehicles use fuel cells to produce electricity, which is then used to power the vehicle. Fuel cells promise a two- to three-fold increase in vehicle efficiency over conventional internal combustion engine vehicles and emit only water vapor in use. Similar to electric vehicles, storing enough hydrogen to obtain sufficient vehicle range before refueling is a challenge. Fuel cells also require a convenient refueling infrastructure, which does not exist today. Durability and costs of fuel cells and hydrogen production also remain challenges. (See Climate TechBook: Hydrogen Fuel Cell Vehicles)
  • Biofuels: Until electric vehicles were widely introduced in 2011, biofuels had been the primary focus of use and research for alternative fuels in the passenger vehicle market. Biofuels used currently include ethanol, biodiesel, and other fuels derived from biomass. To obtain significant reductions in greenhouse gas emissions using biofuels in passenger vehicles, a transition to advanced biofuels (e.g., cellulose for drop-in biofuels) with significantly lower greenhouse gas emission profiles will be required. (See Climate TechBook: Biofuels Overview)

Medium- and Heavy-Duty Vehicles

Medium-duty vehicles have a gross vehicle weight of 8,500 to 26,000 pounds, such as large pick-up trucks and SUVs, small buses, cargo vans, and short-haul trucks. Heavy-duty vehicles have a vehicle weight over 26,000 pounds and are used in both long-distance and local transport. Heavy-duty vehicles include long-haul trucks, large buses, and other vehicles. Medium- or heavy-duty vehicles (e.g., freight and delivery trucks) make up many commercial vehicle fleets; these fleets consume large quantities of fuel because of intensive use and the relatively low fuel economy of their vehicles. (See Climate TechBook: Medium- and Heavy-Duty Vehicles)

Table 1: Medium- and Heavy-Duty Trucks in the United States (2011).

Vehicle classes are defined by vehicle weight. Since Class 3 starts at 10,001 pounds, medium-duty vehicles that weigh between 8,500 and 10,000 pounds are not included here. Class 7-8 vehicles weigh more than 26,000 pounds.

 

Number of Registered Vehicles

Average Annual Miles per Vehicle

Average Fuel Economy (mpg)

Class 3-8 Single-Unit Trucks

7,819,000

13,239

7.3

Class 7-8 Combination Trucks

2,452,000

66,759

5.8

Source: ORNL. 2013. "Transportation Energy Data Book Edition 32." Oak Ridge National Laboratory. Accessed August 14, 2013. http://cta.ornl.gov/data/download32.shtml.

Technology options to reduce fuel consumption and greenhouse gas emissions include the following:

  • Idle reduction: A significant amount of fuel use could be avoided by reducing vehicle idling – an average tractor-trailer spends six hours each day idling to generate electricity for AC and heating systems.[6] Idle reduction technologies include several options. For example, auxiliary power units in vehicles or electrical outlets at truck stops  allow drivers to “plug in” their vehicles to operate the necessary systems. Hybrid drivetrains, similar to those used in passenger vehicles, can also help reduce idling, especially for vehicles used locally in stop-and-go traffic. In the case of buses, idle reduction technologies and strategies have the co-benefit of improving air quality in areas of heavy bus use, such as schools.
  • Vehicle efficiency improvements: Most medium- and heavy-duty vehicles have turbo-charged,[7] direct-injection diesel engines, which are the most energy-efficient internal combustion engines available. State-of-the-art turbo-charged diesel engines achieve 46 to 47 percent efficiency, versus only 25 percent for spark-ignited gasoline engines, which are used in most passenger vehicles in the United States. Options for improving medium- and heavy-duty vehicle efficiency include engine improvements, transmission enhancements, improved aerodynamics and changes in systems and logistics. Overall, existing technology improvements could reduce fuel use by new long-haul tractor-trailers by 18 to 50 percent, with the 50 percent reduction requiring about 5 years of savings to pay off.[8]
  • Low-carbon fuels: These modes can also benefit from alternative fuel use. Lower-carbon fossil fuels, such as natural gas, can reduce conventional air pollutants as well as greenhouse gas emissions.[9] For diesel-powered trucks, blends of up to 20 percent biodiesel can be used in engines without any modification. (See Climate TechBook: Biodiesel)

Aircraft

Aircraft emissions in the United States are about 8 percent of total transportation sector emissions,[10] and are expected to grow significantly in the long term. Business-as-usual projections for aircraft energy consumption growth in the United States are estimated at 0.5 percent per year from 2011 to 2040.[11]

Table 2: U.S. Certificated Air Carrier Fuel Consumption and Travel (2011)

 

Domestic operations

International operations

Aircraft-miles (millions)

6,004

1,777

Fuel consumed (million gallons)

11,071

6,523

Seats per aircraft

114.9

191.5

Aircraft-miles flown per gallon

0.54

0.27

Energy Intensity (Btu/passenger-mile)

2,638

-

Energy Intensity (Btu/vehicle-mile)

269,681

-

Source: U.S. BTS. 2013. "Table 4-8: Certificated Air Carrier Fuel Consumption and Travel." Bureau of Transportation Statistics. Accessed August 15, 2013. http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_statistics/html/table_04_08.html; ORNL. 2013. "Transportation Energy Data Book Edition 32." Oak Ridge National Laboratory. Accessed August 14, 2013. http://cta.ornl.gov/data/download32.shtml.

A number of options are available to limit the growth in aviation greenhouse gas emissions. These include improved navigation systems in the near to medium term and advanced propulsion systems, lightweight materials, improved aerodynamics, new airframe designs, and alternative fuels over the medium to long term.

In the near term (to 2025), the most promising strategies for improving the efficiency of aircraft operations are improvements to the aviation system: advanced communications, navigation, surveillance, and air traffic management, as opposed to changes to aircraft themselves. These improvements have the potential to decrease aircraft fuel consumption and improve aviation operations by shortening travel distances and reducing congestion in the air and on the ground.

Over the longer term (out to 2050), efficiency improvements can be achieved by aircraft technologies including more efficient engines, advanced lightweight materials, and improved aerodynamics. Since aircraft have a much longer lifetime than on-road vehicles (30 to 40 years compared to an average of 14 years for a passenger vehicle in the United States), the fleet-wide penetration of advanced technologies will take a number of years. Early aircraft retirement programs might be able to push more rapid fleet turnover, but the potential benefits of such a program are uncertain.

The potential for fuel switching on jet aircraft is limited in the near future, compared to on-road vehicles. The only feasible options that will reduce greenhouse gas emissions are “drop-in” replacements to petroleum-based jet fuels, which include hydroprocessed renewable jet fuel (from plants or algae) and thermochemically produced Fischer-Tropsch fuels (from biomass or fossil fuel feedstocks, if produced with carbon capture and storage). These fuel production processes are being demonstrated by major carriers today, but are not being produced at commercial scale. Over the longer term, these fuels face numerous challenges with respect to production, distribution, cost, and the magnitude of greenhouse gas benefits.

Marine Transportation

Emissions from marine transportation are about 2 percent of current U.S. transportation emissions, with little domestic growth expected over the next 30 years. On the other hand, due to increases in economic activity and international trade, international marine emissions are estimated to increase by at least 50 percent over 2007 levels by 2050.

Table 3: Domestic Marine Statistics (2011)

Number of Vessels

40,521

Ton-miles (billions)

500

Tons shipped (millions)

888

Average length of haul (miles)

563.5

Source: Department of Energy (DOE), Transportation Energy Data Energy Book 32, 2013. http://cta.ornl.gov/data/chapter9.shtml.

The majority of marine vessels used for commercial operations are powered by highly efficient diesel engines.[12] These engines generally have a longer lifetime than those used in on-road transportation (30 years or more); thus, technical improvements to new engines might not reduce this sector’s emissions in the shorter term.

Immediate reductions in greenhouse gas emissions from marine vessels are available by simply reducing speed. However, reducing speed also reduces shipping capacity. To maintain shipping supply, shippers would have to perform more trips or increase ship utilization (the load factor). Although more trips could increase greenhouse gas emissions, reductions in shipping supply from reduced speeds can also be countered by increasing port efficiency and optimizing land-side intermodal transportation systems, allowing for faster ship turnaround times.

Additional optimization of shipping logistics, routing, and maintenance could reduce greenhouse gas emissions from shipping. These improvements include increased ship utilization (increased load factor), improved and more consistent maintenance practices, optimized ship control, and route planning optimized for current weather conditions and ocean currents.

Technological mitigation options for new ships, aside from alternative fuels and power, include larger ship sizes, hull and propeller optimization, more efficient engines, and novel low-resistance hull coatings. Improvements in engine design include a more flexible design utilizing a series of smaller diesel-electric engines, all optimized for a single speed, that power an electric drive.

Most alternative energy sources currently in use or under development for application in other sectors could be applied in the marine sector as well. Substituting marine diesel oil or liquefied natural gas for heavy fuel oil (i.e., residual fuel oil) currently used in ships can achieve greenhouse gas reductions. Other alternative fuel and power sources, such as biofuels, solar photovoltaic cells, and fuel cells, are longer-term options.

Other Modes

Rail transportation and buses constitute a very small percentage of current transportation sector emissions in the United States, yet growth rates of energy consumption within these modes are expected to be higher than the growth rates of other modes, with the exception of freight trucks. In the future, these modes could make use of technological advances in other sectors, such as improvements in diesel engine efficiency, hybrid technologies, and alternative fuels. For example, many metropolitan transit systems are transitioning to natural gas buses. In 2011, natural gas accounted for 20 percent of fuel consumed by transit buses.[13]

Global Context

Transportation activity and vehicle ownership is expected to grow significantly worldwide over the next several decades. The transportation sector, however, offers some of the greatest potential to alter the growth path in energy consumption, as illustrated by the expected effects of increased fuel economy and greenhouse gas standards for vehicles on overall energy consumption in the United States. Many non-OECD economies are predicted to experience rapid growth in energy consumption as transportation systems are modernized and the demand for personal motor vehicle ownership increases due to higher per capita incomes. Non-OECD transportation energy use is expected to increase by an average of 2.3 percent per year from 2010 to 2040, compared with a decrease by an average of 0.1 percent per year for transportation energy consumption in the OECD countries.[14]

Policy Options

A range of policy options is available for reducing greenhouse gas emissions from these various modes of transportation. Policies can include pricing policies, fuel economy or greenhouse gas emission standards, and funding for technology research and development.

  • Pricing policy options include feebates,[15] carbon pricing, low-carbon fuel subsidies, fuel taxes based on distance traveled, and more.
  • In the United States and worldwide, strengthening fuel economy and greenhouse gas standards has been the main mechanism for improving the efficiency of passenger vehicles. Vehicle fuel economy standards can be expressed in miles per gallon (mpg) or kilometers per liter (km/l). Vehicle emissions standards limit greenhouse gas emissions from a vehicle and are typically expressed as grams of CO2 equivalent per mile (gCO2e/mi).
  • The U.S. Environmental Protection Agency has two programs – SmartWay Tractors and Trailers and the SmartWay Transport Partnership – which are both designed to help truck owners and freight transport operators choose the most efficient vehicles and save energy and lower operating costs through improved logistics.[16]
  • Policies to address greenhouse gas emissions from international aviation and marine shipping are especially challenging, because they are produced along routes where no single nation has regulatory authority. Two broad policy options are available for controlling emissions from international transportation: continuing work under the International Civil Aviation Organization and International Marine Organization to construct an international agreement for addressing these emissions; or assigning responsibility for these emissions to parties for inclusion in national commitments to reducing greenhouse gas emissions.

Related Business Environmental Leadership Council (BELC) Company Activities

Related C2ES Resources

Plug-in Electric Vehicle Dialogue Initiative, 2011-present

Primer on Federal Surface Transportation Authorization and the Highway Trust Fund, 2011

Reducing Greenhouse Gas Emissions from U.S. Transportation, 2011

Federal Vehicle Standards

Greenhouse Gas Emissions from Aviation and Marine Transportation: Mitigation Potential and Policies, 2009

Comparison of Actual and Projected Fuel Economy for New Passenger Vehicles, 2012

Further Reading / Additional Resources

U.S. Department of Energy, Alternative and Advanced Vehicles

U.S. Department of Transportation, National Transportation Statistics

Endnotes


[1] U.S. EIA. 2013. "Annual Energy Outlook 2013." U.S. Energy Information Administration. April. Accessed August 15, 2013. http://www.eia.gov/forecasts/aeo.

[2] Ibid.

[3] Ibid.

[4] Green Car Reports. 2013. 100,000th Plug-In Electric Car In U.S. Sold Today (More Or Less). May 20. Accessed August 15, 2013. http://www.greencarreports.com/news/1084252_100000th-plug-in-electric-car-in-u-s-sold-today-more-or-less.

[5] U.S. EIA. 2013.

[6] By government mandate, long-haul truckers must rest for 10 hours after driving for 11 hours. During the rest periods, truckers might park at truck stops for several hours and idle their engines to provide their sleeper compartments with air conditioning or heating or to run electrical appliances such as refrigerators or televisions.

[7] In turbo-charging, the intake air is compressed with some of the exhaust gas energy, which would otherwise be wasted. Thus, more air can be taken in and more engine power can be produced from a given engine size.

[8] Greene, David, and Steven Plotkin. 2011. Reducing Greenhouse Gas Emissions from U.S. Transportation. Arlington, Virginia: Center for Climate and Energy Solutions.

[9] Ibid.

[10] U.S. EPA. 2013. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011. Washington, DC: U.S. Environmental Protection Agency, 2-26. Accessed July 19, 2013. http://www.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2013-Main-Text.pdf.

[11] U.S. EIA. 2013.

[12] These engines commonly achieve efficiencies near 50 percent, which is higher than most diesel engine applications, since ships typically operate at steady state under high load conditions.

[13] APTA. 2013. "2013 Public Transportation Fact Book." American Public Transportation Association. April. Accessed August 15, 2013. http://www.apta.com/resources/statistics/Documents/FactBook/2013-Fact-Book-Appendix-A.pdf.

[14] U.S. EIA. 2013. "International Energy Outlook." U.S. Energy Information Administration. July 25. Accessed August 15, 2013. http://www.eia.gov/forecasts/ieo

[15] A feebate can be formulated in terms of fuel economy (fuel consumption per unit distance) or greenhouse gas emissions. The manufacturer (or the purchaser) pays a fee for any vehicles produced (or purchased) that are less efficient than the target level for fuel economy or greenhouse gas emissions. The purchasers of any vehicle produced or sold that is more efficient than the target receive a rebate. The value of the fee or rebate can increase in proportion to the divergence from the targeted value. The feebate changes the initial purchase price of a vehicle, which can have a larger impact on consumer decisions than the savings from higher fuel economy alone.

[16] U.S. EPA. 2013. SmartWay. Accessed August 15, 2013. http://www.epa.gov/smartway.