Transportation Modes

Quick Facts

• Transportation activity and vehicle ownership is expected to grow significantly in all countries over the next 50 years. Over the next two decades, passenger vehicle ownership is expected to double worldwide, with most of the increase occurring in non-OECD countries.
• 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 domestic 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 (GHG) 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 Climate TechBook: Transportation Overview).

Of the various transportation modes, passenger vehicles consume the most energy (see Figure 1). GHG emissions mirror energy use by each mode, because all modes use petroleum fuels with similar carbon contents and thus result in corresponding shares of GHG emissions.

Figure 1: Transportation Energy Use by Mode (2008)

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

Source: U.S. Energy Information Administration (EIA). Annual Energy Outlook 2010. Washington, D.C.: EIA. http://www.eia.gov/oiaf/archive/aeo10

Over the next 20 years, analysts expect energy use for rail, aircraft, buses, and freight trucks to grow at higher average annual rates compared to energy use in light-duty vehicles (LDVs); see Figure 2.

Figure 2: Average Annual Growth in Transportation Energy Use by Mode (2009-2035)

Source: Department of Energy (DOE), Annual Energy Outlook 2011 Early Release, December 2010. http://www.eia.doe.gov/forecasts/aeo/index.cfm

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

Passenger Vehicles

Passenger or LDVs 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 GHG emissions within the transportation sector.

Table 1: Passenger Vehicles in the United States.

Source: Department of Transportation, Bureau of Transportation, National Transportation Statistics, 2010. http://www.bts.gov/publications/national_transportation_statistics

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

• The technological improvements for LDVs can be grouped according to application: engine efficiency, transmission, and other improvements, which include vehicle weight reduction, aerodynamic improvements, and reduced rolling resistance. One significant engine efficiency improvement, the hybrid electric vehicle (HEV), has been on the road for over a decade. There is a range of HEVs available today, and they are expected to make up about 5 percent of annual passenger vehicle sales by 2035.[1] In 2010, HEVs made up over 2 percent of the U.S. passenger vehicle market.[2]
• All-electric vehicle (EV) and plug-in hybrid electric vehicle (PHEV) technologies can eliminate or significantly reduce gasoline consumption. PHEVs offer considerable improvements in engine efficiency over HEVs because an electric motor powered by batteries can run the vehicle, which is much more energy efficient than an internal combustion engine (ICE). A PHEV also includes an ICE that can help power the vehicle when the battery pack is depleted. This extends the range of a PHEV to that of a conventional vehicle. General Motors released the first mass-market PHEV, the Chevrolet Volt in 2011. Further, Nissan released the first mass market EV in 2011, the Leaf. Key hurdles for PHEVs and EVs include battery capacity, durability, and cost, as well as the infrastructure needed to charge batteries. The EIA expects plug-in electric vehicles or PEVs (EVs and PHEVs) to make up 2.6 percent of the passenger vehicle market in 2035.[3]
• Hydrogen fuel cell vehicles (FCVs) 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 ICE vehicles and emit only water vapor in use. Similar to PEVs, storing enough hydrogen to obtain sufficient vehicle range before refueling is a challenge, especially given the current lack of a convenient refueling infrastructure. FCVs can partially compensate for this problem by being significantly more efficient than ICE engines, thus requiring the storage of less on-board energy. Durability and costs of fuel cells and hydrogen production also remain challenges.(See Climate Techbook: Hydrogen Fuel Cell Vehicles).
• The passenger vehicle market has been the focus of biofuels use and research thus far. Biofuels used currently include ethanol, biodiesel, and other fuels derived from biomass. To obtain significant reductions in GHG emissions using biofuels in LDVs, a transition to advanced biofuels with significantly lower GHG emission profiles will be required. (See Climate Techbook: Biofuels Overview).

Medium- and Heavy-Duty Vehicles

Medium-duty vehicles (MDVs) 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 (HDVs) have a vehicle weight over 26,000 pounds and are used in both long-distance and local transport. HDVs 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.

Table 2: Medium- and Heavy-Duty Trucks in the United States (2002).

* Does not include trucks between 8,500 and 10,000 pounds.

Source: Department of Energy (DOE), Transportation Energy Data Energy Book 29, 2010. http://cta.ornl.gov/data/tedb29/Edition29_Full_Doc.pdf

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

• 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.[4] Idle reduction technologies can include, for example, auxiliary power units in vehicles or electrical outlets at truck stops that 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.
• Most medium- and heavy-duty vehicles have turbo-charged,[5] direct-injection diesel engines, which are the most energy-efficient ICEs 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 long-haul tractor-trailers by 18 to 50 percent, with the 50 percent reduction requiring about 5 years to pay off.[6]
• 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 GHG emissions.[7] For diesel-powered trucks, biodiesel blends of up to 20 percent biodiesel can be used in engines without any modification. (See Climate TechBook: Biodiesel).
• 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.[8]

Aircraft

Aircraft emissions in the United States are about 10 percent of total transportation sector emissions, and are expected to grow significantly in the long term. Business-as-usual (BAU) projections for aircraft energy consumption growth in the United States are estimated at 0.6 percent per year to 2035.[9]

Table 3: Energy Intensity of Certificated Air Carriers (2008)

Source: Department of Transportation, Bureau of Transportation, National Transportation Statistics, 2010. http://www.bts.gov/publications/national_transportation_statistics

A number of options are available to limit the growth in aviation GHG 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, and surveillance (CNS) and air traffic management (ATM), 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 short term, compared to on-road vehicles. The only feasible options that will reduce GHG emissions are “drop-in” replacements to petroleum-based jet fuels, which include hydroprocessed renewable jet fuel (HRJ) (from plants or algae) and thermochemically produced Fischer-Tropsch (FT) fuels (from biomass or fossil fuel feedstocks, if produced with carbon capture and storage). Neither of these fuel production processes is commercial at this stage, and over the longer term they also face numerous challenges with respect to production, distribution, cost, and the magnitude of GHG benefits.

Marine Transportation

Emissions from marine transportation are about 5 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, under business-as-usual conditions.

Table 4: Domestic Marine Statistics (2008)

Source: Department of Energy (DOE), Transportation Energy Data Energy Book 29, 2010. http://cta.ornl.gov/data/tedb29/Edition29_Full_Doc.pdf

The majority of marine vessels used for commercial operations are powered by highly efficient diesel engines.^[10] 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 emissions in the shorter term.

• Immediate reductions in GHG 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 GHG 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 GHG 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, each optimized for a single speed, that power an electric drive.
• In the marine sector, most alternative energy sources currently in use or under development for application in other sectors could be applied to ships 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 GHG 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 are a very small percentage of current transportation sector emissions in the United States, yet growth rates for energy consumption within these modes are expected to be higher than that for other modes, with the exception of freight trucks. The rail transportation system is used for both freight and passenger travel. Passenger travel includes intercity, transit, and commuter rail systems. Buses are used for transit and intercity travel, as well as for school transportation. 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 2008, natural gas accounted for over 20 percent of fuel consumed by transit buses.[11]

Global Context

Transportation activity and vehicle ownership is expected to grow significantly in all countries over the next 50 years. Over the next two decades, passenger vehicle ownership is expected to double worldwide, with most of the increase occurring in non-OECD countries. The use of air travel and marine shipping is also expected to increase rapidly, with faster growth rates outside of the United States. Many of the 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. Under BAU conditions, non-OECD transportation energy use is expected to increase by an average of 2.7 percent per year from 2007 to 2030, compared with an average of 0.3 percent per year for transportation energy consumption in the OECD countries.[12]

Policy Options

A range of policy options is available for reducing GHG emissions from these various modes of transportation. Policies can include pricing policies, fuel economy or GHG emission standards, and funding for technology R&D.

• Pricing policy options include feebates[13], carbon pricing, low-carbon fuel subsidies, fuel taxes based on distance traveled, and more.
• In the United States and worldwide, vehicle standards have 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 GHG emissions from a vehicle and are typically expressed as grams of CO₂ equivalent per mile (gCO₂e/mi).
• Policies to address GHG 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 GHG emissions.

Related Business Environmental Leadership Council (BELC) Company Activities

• Air Products
• BP
• Cummins
• Daimler
• GE
• Johnson Controls, Inc.
• Shell
• Toyota

Related C2ES Resources

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

Saving Oil and Reducing Greenhouse Gas Emissions through U.S. Federal Transportation Policy, 2011

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

Aviation and Marine Transportation: GHG Mitigation Potential and Challenges, 2009

Policies to Reduce Emissions from the Transportation Sector, 2008

Federal Vehicle Standards

Comparison of Passenger Vehicle Fuel Economy and GHG Emission Standards around the World

Further Reading / Additional Resources

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

U.S. Department of Transportation, National Transportation Statistics