- - -

Greenhouse Gas 

Emissions from the U.S. 

Transportation Sector 

1990–2003

Office of Transportation and Air Quality (6401A) 

EPA 420 R 06 003 

March 2006 

www.epa.gov 
Greenhouse Gas Emissions from the 

U.S. Transportation Sector, 1990-2003 

March 2006 

Prepared by: 

U.S. Environmental Protection Agency 

Office of Transportation and Air Quality 

With support from: 

ICF Consulting 

9300 Lee Highway 

Fairfax, Virginia 22031 
Transportation GHG Emissions Report 

How to Obtain Copies 

You can electronically download this document on the U.S. EPA’s Greenhouse Gas Emissions 

from Mobile Sources web page at: www.epa.gov/otaq/climate.htm. 

For Further Information 

This report was prepared by EPA’s Office of Transportation and Air Quality (OTAQ). Specific 

questions about the report content may be directed to John Davies at (202)564-9467, or 

davies.john@epa.gov. For additional information about climate change and greenhouse gas 

emissions, visit the EPA web site at: www.epa.gov/globalwarming. 

Released for printing: March 2006 

i 
Transportation GHG Emissions Report 

Acknowledgements 

ICF Consulting contributed significantly to this effort. 

The Environmental Protection Agency would like to acknowledge the many individual and 

organizational reviewers whose work has significantly improved this document. These include: 

Leif Hockstad, EPA Office of Atmospheric Programs; Mark Schipper, EIA Office of Energy 

Markets and End Use, Energy Information Administration; Michael Wang, Argonne National 

Laboratory; Mark DeLucchi, Institute of Transportation Studies, University of California at 

Davis; Clare Sierawski, USDOT Office of Safety, Energy and Environment; Maryalice Locke, 

FAA Office of Environment and Energy; Lourdes Maurice, Federal Aviation Administration, 

Office of Environment and Energy; and John Maples, EIA Office of Integrated Analysis and 

Forecasting, Energy Information Administration. 

Preface 

The United States Environmental Protection Agency (EPA) prepares the Greenhouse Gas 

Emissions from the U.S. Transportation Sector report to provide researchers, transportation and 

environmental practitioners, policy makers, and the public with a more complete understanding of 

greenhouse gas (GHG) emissions from the U.S. transportation sector. GHG emissions estimates 

in this report are based on the official U.S. Inventory of Greenhouse Gas Emissions and Sinks. In 

addition, this report highlights factors affecting emissions trends, projections, and emerging 

issues that may affect emissions in the future. It also includes information on the full life-cycle 

GHG emissions associated with transportation, GHG emissions from other mobile sources, and 

uncertainties associated with emissions estimates. 

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Transportation GHG Emissions Report 

TABLE OF CONTENTS 

1 INTRODUCTION.......................................................................................................... 1 

1.1 Background....................................................................................................1 

1.2 Report Organization.......................................................................................1 

2 OVERVIEW OF GREENHOUSE GAS EMISSIONS AND TRANSPORTATION ...................... 3 

2.1 Background on Greenhouse Gases ................................................................3 

2.2 Transportation in the Context of U.S. Greenhouse Gas Emissions ...............5 

2.3 Transportation Sources ..................................................................................7 

2.4 GHG Emissions Trends for Major Transportation Sources...........................7 

3 LIGHT-DUTY VEHICLES – PASSENGER CARS, SUVS, MINIVANS, PICKUP TRUCKS,

AND MOTORCYCLES ................................................................................................ 10 

3.1 Overview......................................................................................................10 

3.2 Factors Affecting Light-Duty Vehicle Emissions........................................11 

4 HEAVY-DUTY VEHICLES—FREIGHT TRUCKS AND BUSES ...................................... 18 

4.1 Heavy-Duty “Freight” Trucks......................................................................18 

4.2 Buses............................................................................................................20 

5 AIRCRAFT................................................................................................................ 21 

5.1 Overview......................................................................................................21 

5.2 Factors Affecting Aircraft Emissions ..........................................................21 

5.3 Other Considerations in Estimating Global Warming Impact.....................23 

6 OTHER NON-ROAD TRANSPORTATION SOURCES..................................................... 24 

6.1 Boats/Ships ..................................................................................................24 

6.2 Rail...............................................................................................................25 

6.3 Pipelines.......................................................................................................26 

7 HFCS FROM MOBILE AIR CONDITIONERS AND REFRIGERATED TRANSPORT ............ 27 

8 NON-TRANSPORTATION MOBILE SOURCES—AGRICULTURAL AND CONSTRUCTION 

EQUIPMENT, RECREATIONAL VEHICLES, AND OTHER ............................................. 29 

9 ESTIMATING TRANSPORTATION GHG EMISSIONS—METHODOLOGY AND 

UNCERTAINTY ......................................................................................................... 31 

9.1 Carbon Dioxide Emissions ..........................................................................31 

9.2 Methane and Nitrous Oxide Emissions........................................................34 

10 LIFECYCLE TRANSPORTATION EMISSIONS............................................................... 36 

10.1 Estimates of Transportation-Related CO2 Emissions ..................................37 

10.2 Other Issues/Next Steps ...............................................................................40 

11 GHG EMISSIONS PROJECTIONS AND EMERGING ISSUES .......................................... 41 

11.1 Projected CO2 Emissions from Transportation............................................41 

11.2 Emerging Issues Affecting Passenger Transportation .................................42 

11.3 Emerging Issues Affecting Freight Transportation......................................44 

11.4 Implications for the Future...........................................................................45 

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Transportation GHG Emissions Report 

12 REFERENCES............................................................................................................ 46 

13 APPENDIX A: SUMMARY OF GHG EMISSIONS FOR TRANSPORTATION AND MOBILE 

SOURCES ................................................................................................................. 49 

14 APPENDIX B: CO2 EMISSIONS FROM VARIOUS COMPONENTS OF THE 

TRANSPORTATION LIFECYCLE (PROPORTION RELATIVE TO DIRECT EMISSIONS) .... 55 

15 ABBREVIATIONS, ACRONYMS, AND UNITS .............................................................. 59 

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Transportation GHG Emissions Report 

LIST OF FIGURES 

Figure 2-1. U.S. Greenhouse Gas Emissions by End-Use Economic Sector, 1990–2003 . 5 

Figure 2-2. 2003 Transportation Greenhouse Gas Emissions, by Source .......................... 7 

Figure 2-3. GHG Emissions by Modes of Transportation,a 1990–2003............................. 8 

Figure 3-1. GHG Emissions from Passenger Cars and Light-Duty Trucks, 1990–2003 

(CO2 Eq.)................................................................................................................... 10 

Figure 3-2. Household Vehicle Fuel Consumption by Mode, 2001................................. 11 

Figure 3-3. Comparison of Percent Growth of U.S. Population, Households, Vehicle 

Trips, and Vehicle Miles Traveled by Households, 1990–2001 .............................. 12 

Figure 3-4. Journey to Work Mode Choice, 1980, 1990, 2000 ........................................ 13 

Figure 3-5. Number of New Light-Duty Vehicles Sold, 1976–2003 ............................... 14 

Figure 3-6. New Light-Duty Vehicle Sales (Market Share) by Size Class, 1976, 1990, and 

2003........................................................................................................................... 15 

Figure 3-7. VMT by Passenger Cars and Light-Duty Trucks, 1990-2003 ....................... 15 

Figure 3-8. Sales-Weighted Fuel Economy of New Light-Duty Vehicles (Combined Car 

and Light-Truck Fleet) by Model Year, 1975–2004 ................................................ 16 

Figure 5-1. Aircraft Passenger Load Factor, 1970–2003.................................................. 22 

Figure 5-2. Average Seat-Miles Traveled Per Gallon of Fuel Consumed, 1970–2003.... 23 

Figure 6-1. Ton-Miles Shipped by Domestic Water Transportation, 1990–2003 ............ 25 

Figure 7-1. HFC Emissions from Mobile Air Conditioners and Refrigerated Transport, 

1990-2003 ................................................................................................................. 27 

Figure 7-2. CFC and HCFC Emissions from Mobile Air Conditioners and Refrigerated 

Transport, 1990–2003............................................................................................... 28 

Figure 8-1. Greenhouse Gas Emissions from Non-Transportation Mobile Sources, 1990 

and 2003.................................................................................................................... 29 

Figure 11-1. EIA Projections of Transportation Energy Demand, High, Base, and Low 

Economic Cases, 2003-2025..................................................................................... 41 

Figure 11-2. Historical and Projected VMT from Gasoline- and Diesel-Electric Hybrid 

Vehicles, 2000–2025................................................................................................. 43 

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Transportation GHG Emissions Report 

LIST OF TABLES 

Table 4-1. 2003 Vehicle Registrations, Vehicle Miles Traveled, and Fuel Use for HeavyDuty Trucks .............................................................................................................. 19 

Table 9-1. Comparison of U.S. GHG Inventory Estimates and Bottom-Up Estimates of 

Table 13-3. Methane Emissions from Transportation Sources, 1990-2003 (Tg CO2 Eq.)51 

Table 13-4. Nitrous Oxide Emissions from Transportation Sources, 1990-2003 (Tg CO2

Table 14-1. CO2 Emissions from Various Components of the Transportation Lifecycle 

Table 14-2. Total CO2 Emissions from Various Components of the Transportation 

CO2 for Selected Transportation Fuels and Sources................................................. 33 

Table 10-1. Elements of the Transportation Lifecycle ..................................................... 37 

Table 13-1. Total GHG Emissions from Transportation Sources (All Gases), 1990-2003 

(Tg CO2 Eq.) ............................................................................................................. 49 

Table 13-2. CO2 Emissions from Transportation Sources, 1990-2003 (Tg)..................... 50 

Eq.)............................................................................................................................ 52 

Table 13-5. HFC Emissions from Transportation Sources, 1990-2002 (Tg CO2 Eq.) ..... 52 

Table 13-6. GHG Emissions from Non-Transportation Mobile Sources (All Gases), 1990-

2003 (Tg CO2 Eq.) .................................................................................................... 53 

Table 13-7. CO2 Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg) 53 

Table 13-8. Methane Emissions from Non-Transportation Mobile Sources, 1990-2003 

(Tg CO2 Eq.) ............................................................................................................. 54 

Table 13-9. Nitrous Oxide Emissions from Non-Transportation Mobile Sources, 1990-

2003 (Tg CO2 Eq.) .................................................................................................... 54 

(Proportion Relative to Direct Emissions)................................................................ 55 

Lifecycle (Tg) ........................................................................................................... 57 

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Transportation GHG Emissions Report 

1 Introduction 

1.1 Background 

Although transportation is a vital part of the economy and is essential for everyday activities, it is 

also a significant source of greenhouse gas (GHG) emissions. In 2003, the transportation sector 

accounted for about 27 percent of total U.S. GHG emissions, up from 24.8 percent in 1990. 

Transportation GHG emissions increased by a larger amount than any other economic sector1 

over this period, growing from 1509.3 Tg CO2 Eq. in 1990 to 1,866.7 Tg CO2 Eq. in 2003, an 

increase of 24 percent.2 GHGs from all other sectors increased by a total of 9.5 percent over the 

same timeframe. Looking forward, transportation GHGs are forecast to continue increasing 

rapidly, reflecting the anticipated impact of factors such as economic growth, increased 

movement of freight by trucks and aircraft, and continued growth in personal travel. According to 

the U.S. Department of Energy (DOE), transportation energy use is expected to increase 48 

percent between 2003 and 2025, despite modest improvements in the efficiency of vehicle 

engines. This projected rise in energy consumption closely mirrors the expected growth in 

transportation GHG emissions.3 

This report was developed by the U.S. Environmental Protection Agency’s (EPA) Office of 

Transportation and Air Quality (OTAQ) to help transportation agencies, the transportation 

industry, researchers, and the public better understand the connection between transportation and 

GHG emissions in the United States. The GHG emissions estimates presented in this report are 

taken from the official GHG Inventory produced by EPA, Inventory of U.S. Greenhouse Gas 

Emissions and Sinks: 1990–2003 (“U.S. GHG Inventory”). As a complement to the U.S. GHG 

Inventory, this report includes additional detail on GHG emissions from transportation and nontransportation mobile sources. It also analyzes factors affecting emissions, uncertainty in the data, 

and emerging issues. 

1.2 Report Organization 

The remainder of this report is organized in the following sections: 

Section 2. Overview of Greenhouse Gas Emissions and Transportation—This section 

provides a brief introduction to specific GHGs and their measurement. It also compares the 

1 The “economic sectors” referred to in this report do not represent official Intergovernmental Panel on Climate 

Change (IPCC) categories. EPA has found it useful to estimate emissions by sectoral categories that are commonly 

used for policy analysis. One method allocates emissions to seven different “economic sectors,” which include 

Electricity Generation, and the non-electricity component of all six other sectors (Transportation, Industry, Agriculture, 

Commercial, Residential and U.S. Territories). The second method distributes the emissions from Electricity 

Generation to the remaining “end use” sectors. For purposes of simplicity, this report uses the second categorization 

when referring to sectoral estimates. 

2 U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. 

Washington, DC. The total transportation sector GHGs cited in this report are slightly lower than the transportation 

sector totals reported in the published Inventory (approximately 0.7 to 0.9 Tg lower from 1990 to 2003). This small 

increment represents “other” non-transportation mobile sources, such as lawn mowers and leaf blowers, which are 

counted as transportation in the published Inventory but not this report. GHG emissions are typically reported in terms 

of CO2 equivalent (CO2 Eq.) in order to provide a common unit of measure, and because CO2 is the most prevalent of 

all GHGs. 

3 U.S. Energy Information Administration, Annual Energy Outlook 2005 with Projections to 2025, Table A2. U.S. 

Department of Energy, Energy Information Administration, Washington, DC. 

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Transportation GHG Emissions Report 

nation’s transportation emissions to other sectors, and discusses variables contributing to the rise 

transportation GHGs. 

Section 3. Light-Duty Vehicles – Passenger Cars, SUVs, Minivans, Pickup Trucks, and 

Motorcycles—This section describes GHG emissions from light-duty motor vehicles, currently 

the largest sources of transportation GHGs. 

Section 4. Heavy-Duty Vehicles—Freight Trucks and Buses—This section addresses 

emissions and trends for heavy-duty vehicles. 

Section 5. Aircraft—This section discusses emissions from aircraft, which are the largest source 

of non-road transportation GHG emissions. 

Section 6. Other Non-Road Transportation Sources—This section characterizes emissions 

from boats and ships, rail, and pipelines. 

Section 7. HFCs from Mobile Air Conditioners and Refrigerated Transport—This section 

describes HFC emissions from transportation sources, which include mobile air conditioners and 

refrigerated transportation units. 

Section 8. Non-Transportation Mobile Sources—This section discusses GHG emissions from 

non-transportation mobile sources, such as agricultural equipment, construction equipment, and 

other utility equipment. 

Section 9. Estimating Transportation GHG Emissions—Methodology and Uncertainty — 

This section briefly describes methods that are used to estimate transportation GHG emissions 

and explores uncertainties in the calculations. 

Section 10. Lifecycle Transportation Emissions—This section examines GHG emissions from 

a broader lifecycle perspective, including activities such as fuel processing and distribution, 

vehicle manufacture and vehicle maintenance. 

Section 11. GHG Emissions Projections and Emerging Issues—This section provides 

forecasts of GHG emissions from transportation sources through 2025 and highlights some of the 

issues affecting trends in GHG emissions from transportation during this time period. 

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Transportation GHG Emissions Report 

2 Overview of Greenhouse Gas Emissions and Transportation 

2.1 Background on Greenhouse Gases 

Greenhouse gases (GHGs) occur naturally in the Earth’s atmosphere and help to keep the planet 

hospitable to life by trapping some of the sun’s natural heat. Without this “greenhouse effect,” the 

Earth’s average surface temperature would be about 33 degrees Celsius cooler than it is 

currently.4 The most important naturally occurring GHGs associated with this phenomenon are 

water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). 

Human activities release GHG emissions and contribute to increasing concentrations of GHGs in 

the atmosphere. CO2 is the predominant GHG emitted by human sources. Like most GHGs, CO2 

is produced both by natural and human activities and can be removed from the atmosphere 

through natural processes.5 However, increased production 

of CO2 by human sources has caused total GHG emissions Global Warming Potentials 

to exceed natural absorption rates, resulting in increased (100-Year Time Horizon) 

atmospheric concentrations. Since the beginning of the Greenhouse Gas GWP

industrial revolution, atmospheric concentrations of CO2 CO2 1 

have increased by nearly 30 percent, CH4 concentrations N2O 296 

have more than doubled, and N2O concentrations have CH4 23 

HFC-125 3,400 risen by approximately 15 percent. Human activities over HFC-134a 1,300

the past 70 years have also produced synthetic chemicals HFC-143a 4,300 

that are greenhouse gases, including chlorofluorocarbons HFC-152a 120 

(CFCs), hydrofluorocarbons (HFCs), perfluorocarbons Source: International Panel on Climate 

(PFCs), and sulfur hexafluoride (SF6). 6 Change, 2000. Third Assessment Report: 

Climate Change 2001. Geneva, Switzerland. 

GHG emissions are typically reported in terms of CO2 

equivalent (CO2 Eq.) to provide a common unit of measure, and because CO2 is the most 

prevalent of all GHGs. Other GHGs are converted into CO2 equivalent on the basis of their global 

warming potential (GWP), which is defined as the cumulative radiative forcing7 effects of a gas 

over a specified time horizon in comparison to CO2 (see sidebar). For example, one kilogram of 

CH4 is estimated to have the same radiative forcing effect as 21 kilograms of CO2. 8 

CO2 accounted for 85 percent of the radiative forcing effect of all human-produced GHGs in the 

United States in 2003. This proportion is higher for transportation sources, with CO2 representing 

about 96 percent of the sector’s GWP-weighted emissions. The transportation sector is the largest 

4 International Panel on Climate Change (IPCC), 2001. Third Assessment Report, Climate Change 2001: A Scientific 

Basis. Cambridge, UK. 

5 International Panel on Climate Change (IPCC), 2001. Third Assessment Report, Climate Change 2001: A Scientific 

Basis. Cambridge, UK. 

6 International Panel on Climate Change (IPCC), 2001. Third Assessment Report, Climate Change 2001: A Scientific 

Basis. Cambridge, UK. 

7 Radiative forcing is the change in balance between radiation entering the Earth’s atmosphere and radiation being 

emitted back into space. A “positive radiative forcing effect” means that the ratio of incoming to outgoing radiation 

increases, generally resulting in a warming of the Earth. Conversely, a “negative radiative forcing effect” generally 

results in cooler Earth temperatures. 

8 Note that the GWPs used in this report are those reported in IPCC’s Second Assessment Report, which is consistent 

with international inventory guidelines. The IPCC has published a Third Assessment Report with revised GWPs, which 

are currently being considered for international inventory guidelines. This report presents estimates of CO2, N2O, CH4, 

and HFC emissions in teragrams or trillion grams of carbon dioxide equivalent (Tg CO2 Eq.) unless noted otherwise. 

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Transportation GHG Emissions Report 

source of domestic CO2 emissions, producing over 30 percent of the nation’s total in 2003. The 

vast majority of anthropogenic CO2 emissions come from the combustion of fossil fuels.9 CO2 

production is related to the amount of fuel combusted and the fuel’s carbon content.10 The U.S. 

transportation sector derived all but 1 percent of its energy from fossil fuels in 2003, 97 percent of 

which was petroleum.11 

CH4 and N2O collectively represented 13 percent of all United States GHGs in 2003, but only 

accounted for 2 percent of the transportation total. These gases are released during fossil fuel 

consumption, although in much smaller quantities than CO2. They are also unlike CO2 in that 

their emissions rates are affected by vehicle emissions control technologies. 

A final category of GHGs comprises various families of synthetic chemicals. These include 

compounds such as CFCs and hydrochlorofluorocarbons (HCFC) that result in stratospheric 

ozone depletion and are controlled under the Montreal Protocol. Because of their required phase 

out, ozone depleting substances are not included in official estimates of national GHGs.12 

Compounds such as HFCs, perfluorocarbons (PFC), and SF6 have been identified as acceptable 

alternatives to ozone depleting substances. Nonetheless, the replacement chemicals are also 

potent greenhouse gases with very high global warming potential. While small quantities of these 

chemicals are released, they accounted for approximately 2 percent of GWP-weighted GHGs 

from all U.S. sectors in 2003. HFCs are the primary replacement chemicals associated with 

transportation sources, replacing CFCs and HCFCs in vehicle air conditioning and refrigeration 

systems. Leakage of HFCs was responsible for 2 percent of transportation GHGs in 2003. 

Transportation sources emit several other compounds that are believed to have an indirect effect 

on global warming but are not considered greenhouse gases. These substances include ozone, 

carbon monoxide, (CO) and aerosols. Scientists have not yet been able to quantify their impact 

with certainty, and these compounds are not included in the transportation GHG emissions 

estimates.13 

9 Approximately 95 percent in the U.S. 

10 A lesser consideration is the fraction of the carbon oxidized, which is assumed to be 100 percent for emissions from 

transportation. The formula for CO2 emissions from fossil fuels is Fuel Combusted X Carbon Content Coefficient X 

Fraction Oxidized X (44/12). 

11 Approximately 2.5 percent is in the form of natural gas, with less than 1 percent renewables (alcohol fuels blended 

with gasoline to make gasohol) and electricity. Source: Oakridge National Laboratory, Transportation Energy Data 

Book, Table 2.2. Citing, U.S. Department of Energy, Energy Information Administration, Monthly Energy Review. 

12 Transportation GHG estimates reflect an accounting issue related to the phase-in of HFCs, primarily as a 

replacement for CFCs in vehicle air conditioners. Transportation HFCs increased from virtually zero in 1990 to over 40 

Tg CO2 Eq. in 2003, at which point they represented about 2 percent of total transportation GHGs. CFCs emissions 

have declined over the same period, but they are not reported in official GHG inventories because of their required 

phase-out under the Montreal Protocol. As a result, the official transportation GHG estimates do not reflect the net 

impact of increasing HFCs and declining CFCs. On balance, the introduction of HFCs has reduced GWP-weighted 

GHG emissions because these substances have lower global warming potential than CFCs. 

13 Ozone traps heat in the atmosphere and prevents a breakdown of CH4, but its lifetime in the atmosphere varies from 

weeks to months, making it difficult to estimate net radiative forcing effects. CO indirectly affects global warming by 

reacting with atmospheric constituents that would otherwise destroy CH4 and ozone. Aerosols are small airborne 

participles or liquid droplets that have both direct and indirect effects on global warming. The most prominent aerosols 

are sulfates and black carbon, or soot. Sulfate aerosols also have some cooling effect by reflecting light back into space. 

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Transportation GHG Emissions Report 

2.2 Transportation in the Context of U.S. Greenhouse Gas Emissions 

Although the United States accounts for approximately 5 percent of the world’s population, it 

produces an estimated 21 percent14 of the world’s GHG emissions, amounting to 6,900 Tg CO2 

Eq. in 2003.15 Transportation sources were responsible for about 27 percent of total U.S. GHG 

emissions in 2003 (1,866.7 Tg CO2 Eq.).16 Non-transportation mobile sources, such as equipment 

used for construction and agriculture, accounted for an additional 2.1 percent of the total U.S. 

GHG emissions (144.8 Tg CO2 Eq.). These estimates are primarily representative of “tailpipe” 

GHGs that result from the use of energy to power vehicles.17 They do not include “lifecycle” 

emissions from processes such as the extraction of crude oil and manufacture of vehicles. 

(Lifecycle issues are discussed in Chapter 10.) 

Figure 2-1. U.S. Greenhouse Gas Emissions by End-Use Economic Sector, 1990–2003 Tg CO2 Eq. 

2,500 

2,000 

1,500 

1,000 

500 

0 

1990 1992 1994 1996 1998 2000 2002 

Industry 

Transportation 

. 

Commercial 

Residential 

Agriculture 

Source: Derived from U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. 

Washington, DC, Table 2-16. 

Note: GHG emissions from electricity generation are distributed to economic sectors. Also, territories are excluded even though they are 

reported in the U.S. inventory. Territories comprise less than 1 percent of national emissions. 

Total U.S. production of greenhouse gases in 2003 was 13 percent greater than in 1990. By 

comparison, transportation GHGs grew almost 24 percent over the same period. GHG emissions 

14 Based on 2000 data reported by the Climate Analysis Indicators Tool (CAIT), World Resources Institute, 

http://cait.wri.org/cait.php. Does not adjust for land-use and forestry change. 

15 U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. 

Washington, DC, Table 2-16. 

16 Based on global warming potential of all gases emitted. 

17 There are two notable exceptions. Included in the transportation estimates are pipelines, which are used as a means 

of transporting petroleum and natural gas. Pipeline GHGs include emissions from natural gas used to operate pumps, 

motors, engines, and compressors, but not electricity used in the operation of pipelines. This is consistent with the 

energy accounting procedures used by the U.S. Energy Information Administration. Second, the transportation sector 

includes the emission of HFCs from vehicle air conditioning and refrigerated transport. This process occurs as a result 

of leakage during equipment operation, servicing, and disposal. Pipeline and HFC emissions collectively accounted for 

slightly less than 3.5 percent of total transportation GHGs. 

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Transportation GHG Emissions Report 

from the transportation sector increased more in absolute terms than any other sector,18 growing 

by 357.4 Tg CO2 Eq. from 1990 to 2003 (Figure 2-1). The growth rate of transportation GHGs 

was equal to the residential sector (also 24 percent), slightly above the commercial sector (22 

percent), and considerably greater than agriculture (3 percent) and industry (which decreased by 2 

percent). 

The overall rise in U.S. GHGs primarily reflects increased emissions of CO2 as a result of 

increasing fossil fuel combustion. Transportation petroleum use grew by 23 percent from 1990 to 

2003 and accounted for 93 percent of the increase in total U.S. petroleum consumption over this 

period. Considering only CO2, transportation sources emitted 1780.7 Tg CO2 in 2003, an increase 

of 319.0 Tg (or 22 percent) from 1990. The combined emissions of CH4 and N2O decreased by 

4.0 Tg CO2 Eq. over the same period, due largely to the introduction of control technologies 

designed to reduce criteria pollutant emissions.19 Meanwhile, HFC emissions from mobile air 

conditioners and refrigerated transport increased from virtually no emissions in 1990 to 42.7 Tg 

CO2 Eq. in 2003 as these chemicals were phased in as substitutes for ozone depleting substances. 

Categorizing U.S. Greenhouse Gas Emissions: The Difference Between 

“Economic Sector” and “End Use Economic Sector” Estimates 

The “economic” sectors referred to in this report do not represent official Intergovernmental Panel on Climate 

Change (IPCC) categories. IPCC guidelines allocate emissions into the following sectors: Energy, Industrial 

Processes, Solvent and Other Product Use, Agriculture, Land Use Change and Forestry, and Waste. However, 

EPA has also found it useful to allocate emissions into “economic sector” categories that are commonly used for 

policy analysis. The 

Inventory of U.S. 

Greenhouse Gas Emissions 

and Sinks classifies 

“economic sector” estimates 

in two different ways. The 

first categorization identifies 

the emissions from 

Electricity Generation, as 

well as the non-electricity 

components of 

Transportation, Industry, 

Agriculture, Commercial, 

Residential, and U.S. 

Territories (note the 

adjacent chart). The second 

categorization distributes 

the emissions from 

Electricity Generation to the 

remaining “end use 

economic sectors” in which 

electricity is consumed. 

The sectoral estimates in 

this report are based on the second categorization. However, if electrical generation is considered a separate 

sector, it accounts for 33 percent of all GHG emissions, and is larger than any of the five sectors discussed in this 

report. Also, emissions from Electricity grew by 24.7 percent from 1990 to 2003, representing the largest 

percentage increase of any economic sector. See the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 

1990–2003 Section 2.2 for additional discussion of the two approaches. Tg C

O2 Eq. 

18 Based on “end use economic sector” estimates, in which emissions from Electricity Generation are allocated to 

economic sectors in which electricity is consumed. 

19 The decline in CFC emissions is not captured in the official transportation estimates. See footnote 9 above. 

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Transportation GHG Emissions Report 

2.3 Transportation Sources 

In 2003, about 81 percent of transportation GHG emissions in the United States came from “onroad” vehicles, including passenger cars, sport-utility vehicles (SUVs), vans, motorcycles, and 

medium- and heavy-duty trucks and buses (Figure 2-2). “Light-duty” vehicles, which are used 

primarily for personal transport, accounted for 62 percent of total transportation emissions. This 

category consists of passenger cars, (35 percent of the transportation total), “light-duty trucks,” 

including SUVs, minivans and pickup trucks (27 percent), and motorcycles (less than 1 percent). 

Heavy-duty vehicles, which include trucks and buses, were responsible for 19 percent of total 

transportation emissions. 

Non-road transportation sources produced 16 percent of all transportation GHG emissions in 

2003. Aircraft were the largest non-road source, producing 9 percent of total transportation 

GHGs. Other non-road sources include boats and ships (3 percent), rail (2 percent), and pipelines 

(2 percent).20 

Finally, the transportation sector estimates include emissions from sources that are classified as 

neither on-road nor non-road. Approximately 2 percent of total transportation emissions in 2003 

consisted of HFCs from vehicle air conditioning and refrigerated transport. Another 1 percent 

came from lubricants, consisting mainly of oil used in motor vehicle engine combustion. 

Figure 2-2. 2003 Transportation Greenhouse Gas Emissions, by Source 

Passenger 

Cars 

35% 

Light Trucks 

27% Lubricants 

1% 

Pipelines 

2% 

Locomotives Heavy-Duty 

2% Vehicles 

Boats and Aircraft 19% 

Ships 9% 

3% 

Source: U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. Washington, 

DC, Table 2-9. 

2.4 GHG Emissions Trends for Major Transportation Sources 

The increase in transportation emissions from 1990 to 2003 reflects continued growth in 

passenger and freight travel, which has substantially exceeded improvements in the energy 

efficiency of most major transport modes. GHG emissions from on-road vehicles increased by 

20 Estimated pipeline GHGs include emissions from natural gas used to operate pumps, motors, engines, and 

compressors, but not electricity used in the operation of pipelines. This is consistent with the energy accounting 

procedures used by the U.S. Energy Information Administration. 

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Transportation GHG Emissions Report 

308.6 Tg CO2 Eq., or 26 percent, from 1990 to 2003. Meanwhile, GHG emissions from non-road 

transportation sources increased by 7.8 Tg CO2 Eq., or 3 percent (Figure 2-3).21 

Figure 2-3. GHG Emissions by Modes of Transportation,a

 1990–2003 

Share of Transportation GHG Emissions 

2,000 100% 

1,800 

1990 2003 

NonRoad 1,600 80% 

1,400 

1,200 60% 

1,000 

On- 800 40% Road 

600 

400 20% 

200 

0 0% 

Source: U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. Washington, 

DC, Table 2-9. 

Note: “Other Non-Road’ includes boats and ships, rail, pipelines, and lubricants. 

a

 Emissions of HFCs from refrigerated transport and mobile air conditioners are not included in this chart. To view HFC emission trends, 

please refer to Table 13-5. 

GHG emissions from light-duty vehicles (passenger cars and light-duty trucks) grew 19 percent 

from 1990 to 2003. The overall rise can be broadly explained by a 34 percent increase in lightduty vehicle miles traveled (VMT) over the period, which outweighed a small improvement in 

overall light-duty fuel economy. However, it is worth noting that the improvement in vehicle 

energy efficiency was due primarily to the replacement of less fuel-efficient vehicles from the 

1970s and early-1980s. Since 1988, the average fuel economy of new light-duty vehicles sold has 

declined as a result of increasing light-duty truck sales. In 2002, sales of new light-duty trucks 

overtook passenger cars. As one primary result, GHGs from light-duty trucks increased by 51 

percent from 1990 to 2003, compared with a 2 percent increase from passenger cars. 

GHG emissions from heavy-duty vehicles (predominantly freight trucks) grew by 57 percent 

from 1990 to 2003—more than twice the rate of light-duty vehicles. An increase in truck freight 

haulage22 caused heavy-duty truck VMT to rise 48 percent over the same period.23 Meanwhile, 

21 This does not include lubricants, which are used for all modes. Lubricant GHG emissions decreased by 1.6 Tg CO2 

Eq. from 1990 to 2003. 

22 According to data from the Commodity Flow Survey and additional estimates compiled by the Bureau of 

Transportation Statistics, the value of goods transported by truck domestically increased by 42 percent, and ton-miles 

increased 56 percent between 1993 and 2002 (survey data are not available for 1990 or 2003). These figures do not 

include “multimodal combinations.” Source: U.S. Department of Transportation, Bureau of Transportation Statistics. 

Freight Shipments in America: Preliminary Highlights from the 2002 Commodity Flow Survey Plus Additional Data. 

Table 1. 

23 Refers to combination trucks and single-unit trucks, excluding two-axle, four-tire trucks. Federal Highway 

Administration, 2004. Highway Statistics 2003. Washington, DC. Tg CO2 Eq. 

Passenger Cars 

Light Trucks 

Heavy-Duty Vehicles 

Aircraft 

Other Non-Road 

1990 1992 1994 1996 1998 2000 2002 

8
Transportation GHG Emissions Report 

overall heavy-duty truck fuel economy declined from 6.0 to 5.7 miles per gallon,24 although the 

average vehicle size has increased slightly and data for this mode is less certain. 

In contrast to on-road vehicles, aircraft GHG emissions decreased by 3 percent from 1990 to 

2003. GHGs from military aircraft declined significantly over the period, while other sources of 

aviation GHGs increased moderately. The largest source of aviation GHGs are commercial 

aircraft, which produced 4.7 percent more greenhouse gases in 2003 than 1990. However, the rise 

in commercial aircraft GHG emissions was significantly less than the growth in air travel, with 

aircraft passenger miles increasing 48 percent over the same timeframe. Emissions per passenger 

mile decreased by 24 percent from 1990 to 2003, representing the most significant improvement 

in emissions intensity of any major mode. Most of the improvement reflected the increasing fuel 

efficiency of aircraft and increased numbers of occupied seats. 

Among other non-road sources, GHG emissions from rail increased 18 percent from 1990 to 

2003. Water-based transportation GHGs appear to have increased similarly (17 percent), although 

the data show much more fluctuation and have a higher degree of uncertainty. (See Section 9.1.2 

for a discussion of uncertainty in GHG emissions estimates.) Pipeline emissions were virtually 

unchanged between 1990 and 2003. 

Impact of Freight Transportation 

Freight GHG emissions25 increased by 46 percent between 1990 to 2003, while GHGs from 

passenger modes increased by 20 percent.26 Collectively, freight sources emitted 13 percent more 

GHGs per ton-mile in 2003 than in 1990. Most of the increase in GHG intensity resulted from a 

shift to energy-intensive freight modes. Rail is typically the least energy-intensive freight mode. 

Measured in BTUs per ton-mile, rail used 90 percent less energy than trucks and 80 percent less 

than ships.27 While the share of freight carried by rail has remained roughly constant, trucks’ 

share of freight ton-miles increased from 26 percent in 1993 to 32 percent in 2002,28 accounting 

for most of the overall increase in freight GHG output and intensity.29 

24 According to FHWA’s estimates of average fuel economy, based on estimates of fuel consumption and VMT by 

vehicle type. Federal Highway Administration, 1997. Highway Statistics: Summary to 1995. Washington, DC, Table 

VM-201A, and Highway Statistics 2003, Table VM-1. 

25 Freight modes are those used to ship materials and goods, and include heavy-duty trucks, freight rail, freight vessels, 

and pipelines. Emissions from refrigerated transport are also freight-related and so are allocated to freight 

transportation. 

26 The U.S. GHG Inventory does not explicitly calculate aircraft emissions resulting from shipping materials and 

goods. These emissions are generally included in overall estimates for “commercial aircraft,” which are categorized as 

passenger transport. 

27 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, 2004. Transportation Sector Trend 

Data. Available at http://intensityindicators.pnl.gov/data.html. 

28 Data not available for 1990 or 2002. U.S. Department of Transportation, Bureau of Transportation Statistics, and 

U.S. Department of Commerce, U.S. Census Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow 

Survey. Washington, DC. 

29 Air shipments required approximately 82 times more energy per ton-mile than rail in 2001, according to the DOE 

estimates referenced above. While air was the fastest-growing mode of freight transport, its share of total shipments 

remained below 1 percent. 

9
Transportation GHG Emissions Report 

3 Light-Duty Vehicles – Passenger Cars, SUVs, Minivans, Pickup 

Trucks, and Motorcycles 

3.1 Overview 

Light-duty vehicles30 in the U.S. produced 1152.6 Tg CO2 Eq. in 2003, representing 77 percent of on-road 

vehicle GHG emissions and 62 percent of total transportation emissions.31 GHG emissions from lightduty vehicles increased 19 percent between 1990 and 2003. CO2 emissions increased 20 percent, or 187.8 

Tg, mirroring the growth in light-duty vehicle fuel consumption. Meanwhile, emissions of CH4 and N2O 

from light-duty fleet vehicles decreased by 2.1 and 2.9 Tg CO2 Eq., due to the introduction of vehicle 

emissions control technologies in newer vehicles. 

A growing portion of new vehicle sales from 1990 to 2003 consisted of light-duty trucks, which include 

pickup trucks, SUVs, and vans. GHG emissions from light-duty trucks increased 51 percent from 1990 to 

2003, while emissions from passenger cars increased about 2 percent (Figure 3-1). In 2003, light-duty 

trucks produced 43 percent of light-duty vehicle GHG emissions, up from 34 percent in 1990. 

Motorcycles make up a very small proportion of light-duty GHG emissions (less than 0.1 percent in 

2003), and this share has remained relatively constant. 

Figure 3-1. GHG Emissions from Passenger Cars and Light-Duty Trucks, 1990–2003 (CO2 Eq.) 

Share of On-Road 

Tg CO2 Eq. 

1,200 

1,000 

800 

600 

400 

200 

0 

Passenger 

Cars 

Light-duty trucks 

1990 1992 1994 1996 1998 2000 2002 

100% 

80% 

60% 

40% 

20% 

0% 

GHG Emissions 

1990 2003 

Source: Derived from U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. Washington, 

DC, Table 2-9, with adjustments. 

Light-duty vehicles are primarily used for personal transportation, although some are used for business 

purposes, or are maintained as part of public sector or private sector fleets.32 Among vehicles owned by 

households, light-duty trucks account for an even higher share of light-duty GHG emissions. Nearly half 

of household vehicle fuel consumption was by light-duty trucks in 2001, including vans (22 percent), 

SUVs (17 percent), and pickup trucks (11 percent).33 Passenger cars consumed about 49 percent of the 

30 Light-duty vehicles are defined as vehicles with a gross vehicle weight rating (GVWR) of less than 8,500 lbs. 

31 This figure does not include motorcycle emissions. 

32 Such uses include rental cars, taxis, police vehicles, and government vehicles. 

33 National Household Travel Survey, 2001. 

10
Transportation GHG Emissions Report 

motor vehicle fuel used by households, and the remaining 1 percent was used by other light-duty trucks, 

recreational vehicles (RVs), and motorcycles.34 

Figure 3-2. Household Vehicle Fuel Consumption by Mode, 2001 

Other Light-Duty 

Vehicles, RVs, 

and Motorcycles 

1% 

Vans

22%

Passenger Cars 

49% 

SUVs

17%

Pickup Trucks 

11% 

Source: Energy Information Administration, 2005. Household Vehicles Energy Use: Latest Data & Trends. Washington, DC. Table A1, p. 54. Available 

online at http://www.eia.doe.gov/emeu/rtecs/nhts_survey/2001/tablefiles/table-a01.pdf. 

3.2 Factors Affecting Light-Duty Vehicle Emissions 

The increase in GHG emissions from light-duty vehicles reflects 1.) growth in vehicle travel and 2.) 

limited improvement in vehicle fuel economy, largely associated with the increased sales and use of lightduty trucks. Light-duty vehicle fuel consumption increased 22 percent between 1990 and 2003,35 

resulting in a 20 percent increase in CO2 emissions. CH4 and N2O emissions also have been influenced by 

the increase in travel, although their potential growth has been mitigated by technologies in newer 

vehicles that have reduced GHG emissions. 

3.2.1 Increasing VMT 

Nationally, travel by light-duty vehicles rose 34 percent between 1990 and 2003. VMT has grown more 

than twice as fast as population, with economic, social, and land use factors spurring increased vehicle 

trip making and VMT per person. According to data from the National Household Travel Survey 

(NHTS),36 VMT by households (which are a subset of all vehicle users) increased by 35 percent between 

1990 and 2001, while the total number of households in the United States increased by only 15 percent 

(Figure 3-3).37 

34 Energy Information Administration, 2005. Household Vehicles Energy Use: Latest Data & Trends. Washington, DC. Table 

A1, p. 54. Available online at http://www.eia.doe.gov/emeu/rtecs/nhts_survey/2001/tablefiles/table-a01.pdf. 

35 U.S. Department of Energy, 2004, Transportation Energy Data Book, Edition 24. Washington, DC. 

36 The predecessor survey of the NHTS is the Nationwide Personal Travel Survey (NPTS). 

37 Federal Highway Administration, 2001 National Household Travel Survey. Washington, DC. “Summary Statistics on 

Demographic Characteristics and Total Travel 1969, 1977, 1983, 1990, and 1995 NPTS, and 2001 NHTS” and “Americans and 

Their Vehicles,” presentation by Pat Hu, Director, Center for Transportation Analysis, Oak Ridge National Laboratory. Available 

online at http://nhts.ornl.gov/2001/html_files/trends_ver6.shtml and 

http://nhts.ornl.gov/2001/presentations/americanVehicles/index.shtml. 

11
Transportation GHG Emissions Report 

Figure 3-3. Comparison of Percent Growth of U.S. Population, Households, Vehicle Trips, and 

Vehicle Miles Traveled by Households, 1990–2001 

40% 

35% 

35% 

30% 

25% 

21% 

20% 

15% 16% 

15% 

10% 

0% 

5% 

Households U.S. Population Vehicle Trips Vehicle Miles Traveled 

Source: Federal Highway Administration, 2001 National Household Travel Survey. Washington, DC. “Summary Statistics on Demographic 

Characteristics and Total Travel 1969, 1977, 1983, 1990, and 1995 NPTS.” 

The rapid increase in VMT by households has been influenced in part by the increasing ownership of 

personal vehicles. The proportion of households without a motor vehicle dropped from 9.2 percent in 

1990 to 7.9 percent in 2001, continuing a longer term pattern of increasing vehicle ownership. In 1969, 

about 20.6 percent of households owned no vehicles. By 2001, more households owned four or more 

vehicles (8.5 percent) than owned no vehicles.38 The most substantial changes in vehicle ownership 

occurred during the late 1960s through 1990, a period when a significant number of women entered the 

workforce, the number of licensed drivers increased rapidly, and disposable income grew. The average 

household in 1969 had 3.16 persons and 1.16 vehicles. Average household size in 1990 dropped to 2.56 

persons, while the number of vehicles increased to 1.77 per household, exceeding the number of licensed 

drivers per household. In 2001, there were about 203.9 million household vehicles serving 190.3 million 

licensed drivers.39 

As vehicle ownership has increased, average vehicle occupancy has declined and the number of vehicle 

trips has grown. According to the Census Bureau’s Journey to Work Survey, the proportion of commute 

trips taken by single-occupant vehicle increased from 73.2 percent in 1990 to 75.7 percent in 2000, while 

carpooling, transit, and walking mode shares declined. These trends reflect longer-term changes in 

commuting patterns. In 1980, 64.4 percent of commuters drove to work alone, while nearly 20 percent 

carpooled, about 6 percent used public transit, and nearly 6 percent walked. By 2000, the proportion using 

carpools had fallen to 12 percent, transit to about 5 percent, and walking to 3 percent (Figure 3-4).40 

Shared use of vehicles also has declined for other forms of personal travel, due in part to smaller 

household sizes and increased vehicle availability. Across all trip purposes, the average number of 

occupants per vehicle in 2000 was 1.6 persons, down from 1.9 in 1977. 

38 Federal Highway Administration, 1999. Summary of Travel Trends: 1995 Nationwide Personal Transportation Survey. 

Washington, DC, Table 16, p. 28; and Federal Highway Administration. 2001 National Household Travel Survey online tool. 

Available online at http://nhts.ornl.gov/2001/index.shtml. 

39 Household vehicles are a subset of all light-duty vehicles and do not include business fleets or vehicles maintained by persons 

living in institutions, such as colleges and military bases. Federal Highway Administration, 2001 National Household Travel 

Survey. Washington, DC. “Summary Statistics on Demographic Characteristics and Total Travel 1969, 1977, 1983, 1990, and 

1995 NPTS, and 2001 NHTS.” Available online at http://nhts.ornl.gov/2001/html_files/trends_ver6.shtml. 

40 Although work trips make up only about one-quarter of total vehicle trips, they are important because work trips tend to 

involve longer distances than trips for other purposes and often are conducted as part of a chain of trips. 

12
Transportation GHG Emissions Report 

Figure 3-4. Journey to Work Mode Choice, 1980, 1990, 2000 

80% 

73% 76% 

70% 64% 

60%

50%

40%

30%

20% 

20% 13%12% 6% 6% 10% 5% 5% 4% 3% 3% 4% 4% 

0% 

1980 

1990 

2000 

Drove Alone Carpooled Public Transit Walk/Bicycle Work at 

Home/Other 

Source: U.S. Decennial Census, Supplemental Survey: Journey-to-Work, various census years, 1960 to 2000, as tabulated by Alan Pisarski and 

reported in A. Pisarski, Commuting in America III. Washington, DC: Eno Transportation Foundation, 2003. 

The average household in 2001 took more than 100 additional vehicle trips per year compared to 1990.41 

In addition to the increase in trip-making, the average household trip length also increased from 8.7 miles 

in 1990 to 9.7 miles in 2001.42 Consequently, the average household traveled over 3,000 more vehicle 

miles in 2001 (21,253 vehicle miles) compared to 1990 (18,161 vehicle miles). 

3.2.2 Changes in Vehicle Fleet Composition and Limited Improvements in Fuel 

Economy 

Over the past 25 years, consumer preferences for new vehicles have shifted notably, with an increasing 

share of buyers opting to purchase light-duty trucks instead of passenger cars (Figure 3-5). In 1976, 

approximately four new passenger cars were sold for each new light-duty truck. By 1990, the ratio had 

shifted to two-to-one (67.1 percent passenger cars and 32.9 percent light-duty trucks), and in 2002, sales 

of light-duty trucks surpassed those of passenger cars. 

41 Federal Highway Administration, 2004. Summary of Travel Trends: 2001 National Household Travel Survey. Table 6. 

42 However, the trend before 1990 is inconsistent. Calculated using Federal Highway Administration, 2001. 2001 National 

Household Travel Survey. Washington, DC. Summary Statistics on Demographic Characteristics and Total Travel 1969, 1977, 

1983, 1990, and 1995 NPTS, and 2001 NHTS. Available online at http://nhts.ornl.gov/2001/html_files/trends_ver6.shtml. 

13
Transportation GHG Emissions Report 

Figure 3-5. Number of New Light-Duty Vehicles Sold, 1976–2003 

12 

Passenger Cars 

Light-Duty Trucks 

1976 1980 1984 1988 1992 1996 2000 

Number of Vehicles Sold

10 

8 

6 

4 

2 

0 

(millions) 

Source: U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Washington, DC. Tables 4.7 and 4.8. 

Most of the growth in the light-truck market has been led by rapidly increasing sales of SUVs. As a result, 

the SUV market share has increased from less than one percent of new light-duty vehicles in 1976 to 

more than 25 percent of the market by 2003. Other light-duty trucks categories also gained market share 

over the same period, with minivans growing from 5 percent to 8 percent of new vehicles, and pickup 

trucks increasing from about 15 percent to 17 percent43 (Figure 3-6). Meanwhile, the share of new lightduty vehicle sales that are passenger cars has fallen from over 80 percent of the new vehicle market in 

1976 to just over 47 percent in 2003. The total number of passenger cars on the road has declined from 

137.6 million in 2001 to 135.7 million in 2003, while the number of registered light-duty trucks increased 

from 84.2 million to 87.0 million. 44 

43 U.S. Environmental Protection Agency, 2004. Light-Duty Automotive Technology and Fuel Economy Trends, 1975 through 

2004. Washington, DC. 

44 Federal Highway Administration, Highway Statistics 2003 and Highway Statistics 2001. Table VM-1. 

14
Transportation GHG Emissions Report 

Figure 3-6. New Light-Duty Vehicle Sales (Market Share) by Size Class, 1976, 1990, and 2003 

SUVs, 0.5% 

Cars, 80.1% 

Cars, 67.2% 

Cars, 47.2% 

Pickup Trucks, 

16.4% 

Pickup Trucks, 

17.3% 

Vans, 5.0% 

Vans, 9.7% 

Vans, 8.5% 

SUVs, 6.8% 

Pickup Trucks, SUVs, 27.0% 14.5% 

1976 1990 2003 

Source: U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Washington, DC, Table 4.9. Note: Original source percentage 

shares do not equal exactly 100%. 

The growing number of light-duty trucks on the road has corresponded with an increase in light-duty 

truck travel. Light-duty truck VMT increased by 74 percent from 1990 to 2003, while passenger car VMT 

increased by 18 percent ( 

Figure 3-7). In 2003, light-duty trucks comprised about 37 percent of all VMT by light-duty vehicles, up 

from 29 percent in 1990.45 The increasing share of VMT by light-duty trucks is significant because those 

vehicles tend to be less fuel efficient than their passenger car counterparts. 

Figure 3-7. VMT by Passenger Cars and Light-Duty Trucks, 1990-2003 

1990 2003 

Source: Federal Highway Administration, U.S. Department of Transportation, 2004. Highway Statistics 2003. Washington, DC, Table VM-1, and 

Highway Statistics Summary to 1995. Table VM-201. 

45 Oak Ridge National Laboratory, Light-Vehicle MPG and Market Shares System, 2002, as cited in U.S. Department of Energy, 

2003. Transportation Energy Data Book, Edition 23. Washington, DC, and U.S. Environmental Protection Agency, 2004. LightDuty Automotive Technology and Fuel Economy Trends, 1975 through 2004. Washington, DC. 

Passenger Cars 

(18% Increase) 

Light-Duty Trucks 

(74% Increase) 

0 

1 

2 

3 

Vehicle Miles Traveled

(millions) 

15
Transportation GHG Emissions Report 

The Impact of Corporate Average Fuel Economy Standards 

Since 1978, Congress has set standards for the average fleet fuel economy of new cars through the 

Corporate Average Fuel Economy (CAFE) program. These standards initially required a fleet average of 

at least 18 miles per gallon (mpg) for new passenger cars. The required average was subsequently 

increased nearly every year until 1990, when it reached 27.5 mpg. The passenger car requirement does 

not cover light-duty trucks, which must meet their own CAFE standards. The light-duty truck fuel 

economy requirement is lower than the passenger car standard and has been increased more slowly, from 

17.5 mpg in 1982 to 20.7 mpg in 1996.46 The reported fuel economy of both passenger cars and lightduty trucks has closely mirrored CAFE standards. The most significant improvements in passenger car 

fuel economy were reported between 1978 and 1985, and there were moderate increases in new light-duty 

truck fuel economy through the late 1980s. Since then, the fuel economy of both new passenger cars and 

new light-duty trucks has been relatively flat. 

As a result of the increasing market share of light-duty trucks, the sales-weighted fuel economy of new 

light-duty vehicles has steadily declined from its peak of 22.1 mpg in 1987 and 1988 (Figure 3-8). By 

model year 2004, the new light-duty vehicle average had declined to 20.8 mpg. 

Figure 3-8. Sales-Weighted Fuel Economy of New Light-Duty Vehicles (Combined Car and LightTruck Fleet) by Model Year, 1975–2004 Miles Per Gallon 

Peaked at 22.1 in 1987 and 1988 

20 

15 

10 

5 

0 

1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 

Model Year 

Source: U.S. Environmental Protection Agency, 2005. Light-Duty Automotive Technology and Fuel Economy Trends, 1975 through 2004. Washington, 

DC. 

Note: This graph represents the estimated sales-weighted fuel economy of new vehicles, based on EPA’s adjusted estimates for 55/45 combined 

city/highway driving. 

46 Standards for light trucks were initially set in 1978, with separate standards for two-wheel drive and four-wheel drive vehicles. 

16
Transportation GHG Emissions Report 

While EPA-rated new light-duty vehicle fuel economy Effect of Light-duty Vehicle Technologies on 

has declined since the late 1980s, average fuel Criteria Air Pollutants 

economy for the in-use fleet of all light-duty vehicles Criteria pollutants are not included in transportation 

has increased. Most of the gain occurred in the early GHG totals but are considered to be indirect 

1990s, reflecting the retirement of a large number of greenhouse gases and are reported to the United 

Nations Framework Convention on Climate Change 

less fuel efficient vehicles built in the late 1970s and (UNFCCC). Increasingly stringent vehicle emissions 

early 1980s. Overall fuel economy of the in-use fleet controls have dramatically reduced criteria pollutants 

fuel increased from 18.9 mpg in 1990 to 19.6 mpg in from light-duty motor vehicles. Between 1970 and 

2002, during a time when light-duty VMT increased 

1991, then fluctuated and rose slowly to 20.3 mpg in by 32 percent, light-duty vehicle emissions of CO 

2003, representing a 7 percent improvement from dropped by 38 percent, volatile organic compounds 

(VOCs) dropped by 48 percent, and oxides of 1990 levels.47,48 However, the increase in vehicle nitrogen (NOx) fell by 33 percent. 

energy efficiency was offset by a 34 percent increase 

in light-duty VMT from 1990 to 2003, accounting for Source: U.S. Environmental Protection Agency, Emission 

Factors and Inventory Group, Office of Air Quality Planning and the overall growth in light-duty CO2 emissions. Standards, National Emission Inventory (NEI) Air Pollutant 

Emission Trends web site. 

<www.epa.gov/ttn/chief/trends/index.html> 

3.2.3 Improvements in Vehicle Technologies and Emission Controls 

Changes in light-duty vehicle technologies have not significantly impacted CO2 emissions. For the most 

part, these technologies have been used to improve vehicle power, safety, and driving performance, rather 

than to increase vehicle fuel economy.49 By contrast, vehicle emissions control technologies have reduced 

emissions of CH4 and N2O between 1990 and 2003, although the two gasses have been affected 

differently by various generations of these technologies. 

Emissions control technologies were primarily designed to reduce emissions of criteria air pollutants 

under EPA emissions standards. Beginning in the 1970s, auto manufacturers switched from non-catalyst 

control systems to early oxidation catalysts and then to Tier 0–compliant technologies in the mid-1980s. 

Tier 0 technologies were intended to control emissions of the hydrocarbons CO and NOx, but had the cobenefit of reducing CH4 (which is also a hydrocarbon). However, they also increased the amount of N2O 

emitted per mile, and caused overall N2O emissions from light-duty vehicles to grow 27 percent between 

1990 and 1998. 

Newer generation technologies have lowered N2O emission rates. These include Tier 1-compliant 

technologies, introduced in 1994, and low emissions vehicles (LEVs), which entered the fleet in 1996. 

Nonetheless, these emissions rates are still higher than with the non-catalyst control systems. Since these 

newer vehicles entered the fleet, light-duty emissions of N2O have declined to nearly 8 percent below 

their 1990 level. These new vehicle technologies have had a larger impact on CH4 emissions rates from 

light-duty vehicles, which decreased by 52.5 percent from 1990 to 2003 despite a significant increase in 

VMT. 

47 Federal Highway Administration, 1997. Highway Statistics: Summary to 1995. Washington, DC, Table VM-201A, and 

Highway Statistics (annual editions), Table VM-1. FHWA estimates average fuel economy for the in-use fleet from fuel 

consumption data (based on state fuel tax receipts) and estimates of VMT (based on travel monitoring data from states). 

48 For instance, studies of 1984 model vehicles found maximum fuel efficiency achieved at steady-state speeds of 35 to 40 mph, 

while newer studies of 1997 model vehicles showed peak fuel efficiency at steady-state speeds of 50 to 55 mph, and less fuel 

economy loss above 55 mph (U.S. Department of Energy, Transportation Energy Data Book. Edition 23, Table 4.24). 

49 The introduction and growing market share of gas-electric hybrid vehicles is increasing the number of fuel-efficient vehicles 

on the road. A hybrid vehicle uses an electric motor in combination with a traditional combustion engine to power the vehicle 

using less fuel. For instance, according to EPA fuel economy estimates, a 2003 Honda Civic Hybrid gets 47 combined mpg, 

compared to 34 combined mpg with the traditional Honda Civic. The 2005 Ford Escape SUV hybrid gets 33 MPG, while the 

traditional gasoline engine Escape achieves 23 mpg. 

17
Transportation GHG Emissions Report 

4 Heavy-Duty Vehicles—Freight Trucks and Buses 

Heavy-duty vehicles are the second-largest source of U.S. transportation GHGs,50 accounting for 19 

percent of the transportation total in 2003, or 343 Tg CO2 Eq. Heavy-duty vehicles consist of mediumand heavy-duty trucks used primarily for freight haulage (97 percent of heavy-duty emissions) and buses 

(3 percent). Emissions from heavy-duty sources grew by 57 percent between 1990 and 2003, the largest 

increase of any major transportation source. By comparison, emissions from passenger “light-duty” 

vehicles increased 19 percent. Heavy-duty vehicles accounted for 23 percent of the on-road total GHG 

emissions in 2003, up from 18 percent in 1990. 

The majority of heavy-duty trucks run on diesel, while a smaller percentage run on motor gasoline.51 

(Light-duty vehicles generally use motor gasoline.) It should be noted that the U.S. GHG Inventory’s CO2 

calculations for heavy-duty sources are based on diesel estimates from the Energy Information 

Administration, which are lower than estimates compiled from Federal Highway Administration (FHWA) 

and industry sources. As a result, the U.S. GHG Inventory may underestimate the CO2 emissions from 

heavy-duty vehicles. (See Section 9 for discussion of uncertainty.) 

4.1 Heavy-Duty “Freight” Trucks 

Data for heavy-duty trucks are less certain than that of light-duty vehicles, but generally indicate a 

significant increase in activity and constant or slightly declining fleetwide fuel economy. Heavy-duty 

truck ton-miles increased 55.5 percent between 1993 and 2002 to 1.45 trillion ton-miles.52 VMT grew by 

a more modest 48 percent between 1990 and 2003,53 most likely reflecting improvements in distributional 

efficiency. Nevertheless, overall GHG intensity of truck shipments increased 1 percent from 1990 to 2003 

as total fleet fuel economy edged downward over the period. In 2001, trucks required 11 times more 

energy to carry a ton-mile than rail, and 2.2 times more than ships.54 The relative GHG intensity of truck 

haulage has significantly impacted total freight GHG emissions, especially as trucks’ share of total tonmiles increased from 19 percent in 1980 to 26 percent in 1993 and 32 percent in 2002.55 

50 Heavy-duty vehicles are defined by EPA as vehicles weighted over 8,500 pounds. Vehicles weighing between 8,500 and 

10,000 pounds are sometimes considered to be medium-duty trucks, but for simplicity, this category is referred to simply as 

“heavy-duty vehicles.” In addition, to freight trucks, heavy-duty trucks encompass larger commercial vehicles that are not used to 

transport goods, such as some utility, service, and construction vehicles in addition to those used by some households. 

51 A very small portion, consisting mainly urban delivery vehicles, run on alternative fuels. 

52 U.S. Department of Transportation, Bureau of Transportation Statistics, and U.S. Department of Commerce, U.S. Census 

Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow Survey. Washington, DC. 

53 Refers to combination trucks and single-unit trucks, excluding two-axle, four-tire trucks. Federal Highway Administration, 

2004. Highway Statistics 2003. Washington, DC. 

54 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, 2004. Transportation Sector Trend Data 

Available online at http://intensityindicators.pnl.gov/data.html. 

55 U.S. Department of Transportation, Bureau of Transportation Statistics, and U.S. Department of Commerce, U.S. Census 

Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow Survey. Washington, DC. 

18
Transportation GHG Emissions Report 

4.1.1 Factors Underlying Increase in Trucking Activity 

A number of factors contributed to increased freight movement, including the growth in domestic 

consumption and global trade, as well as declining oil costs. Trucks carried a larger share of the 

increasing domestic freight load for several reasons:56 

Changing composition of shipments. The United States and world economies shifted toward more highvalue, low-weight products, such as electronic, electrical, office equipment, pharmaceuticals, and food 

products, which are more conducive to haulage by trucks than by rail or ships. 

Just-in-time inventory practices. Manufacturers that employ just-in-time systems strive to minimize onsite inventory by coordinating their supply deliveries with production schedules. This requires smaller, 

more frequent, and more reliable inbound shipments—characteristics that typically favor trucking over 

rail and also may diminish the loads of smaller trucks. 

Declining labor costs. The costs of truck freight transport have decreased relative to other shipping 

modes, in part because of stiff price competition that followed trucking deregulation with the 1980 Motor 

Carrier Act. 

Manufacturing and warehouse location patterns. Manufacturing and warehousing have migrated from 

urban areas to suburban or rural locations that often provide greater highway access and cheaper land and 

labor. Longer hauls by truck carriers are required to connect more distant supply, production, and 

consumption facilities. At the same time, these facilities are increasingly inaccessible by rail. 

4.1.2 Fuels, Energy Efficiency, and GHG Emissions 

Overall fuel economy for heavy-duty trucks fell from 6.0 mpg in 1990 to 5.7 mpg in 2003.57 These 

calculations are somewhat uncertain and may reflect changes in the average size and weight of trucks or 

other factors. According to the 1992 Truck Inventory and Use Survey (TIUS) and 1997 Vehicle Inventory 

and Use Survey (VIUS), there was a small increase in mean fuel economy for most sizes of heavy-duty 

trucks between 1992 and 1997. 

Although single-unit trucks outnumber combination trucks by more than two-to-one, combination trucks 

account for 64 percent of heavy-duty truck VMT and 72 percent of truck fuel use (Table 4-1). 

Combination trucks are typically much larger than single-unit models. They are also used for much longer 

hauls and require more fuel per mile. Diesel fuel is generally used in combination trucks, while both 

gasoline and diesel are commonly used by single-unit vehicles. 

Table 4-1. 2003 Vehicle Registrations, Vehicle Miles Traveled, and Fuel Use for Heavy-Duty Trucks 

Type of Truck # of Vehicles % VMT (millions) % Fuel Use 

(million gallons) 

% 

Single-Unit 5,666,933 72% 77,562 35% 10,690 28% 

Combination 2,245,085 28% 138,322 64% 26,895 72% 

Source: Federal Highway Administration, U.S. Department of Transportation, 2004. Highway Statistics 2003. Washington, DC. Table VM-1. 

Overall, the share of heavy-duty vehicles using diesel has increased over the period 1990 to 2003. During 

this time period, VMT by diesel-powered heavy-duty trucks increased 60 percent, while VMT by 

gasoline-powered heavy-duty vehicles increased 7 percent.58 

56 For a discussion of these issues, see A Guidebook for Forecasting Freight Transportation Demand, NCHRP Report 388, 

Transportation Research Board, 1997. 

57 Federal Highway Administration, U.S. Department of Transportation, 2004. Highway Statistics 2003. Washington, DC. Based 

on estimates of fuel consumption and VMT by vehicle type 

58 Vehicle miles of travel by alternative fuel heavy-duty vehicles increased by 82 percent for trucks but still comprise a very 

small portion (about 1 percent) of total VMT. 

19
Transportation GHG Emissions Report 

4.2 Buses 

Buses produced approximately 0.5 percent of total transportation GHGs and 0.6 percent of on-road 

emissions in 2003. Bus GHGs increased about 15 percent from 1990. Best estimates suggest that transit 

buses produced about 46 percent of total bus GHGs, followed by schoolbuses at 38 percent, and intercity 

buses at 16 percent. 59 

Transit bus VMT60 increased 45 percent between 1990 and 2002, growing from 1.67 billion to 2.43 

billion vehicle miles.61 The number of schoolbuses in service is estimated to have risen by 21 percent 

over the same timeframe, increasing from approximately 508,000 to 617,067.62 Intercity buses 

passenger-miles and energy use also increased over this period.63 

Most buses run on diesel, while a small portion run on gasoline and alternative fuels. Alternative fuels are 

playing an increasingly significant role in bus travel. Between 1990 and 2003, VMT for buses running on 

alternative fuels increased by 273 percent. Alternative fuels include biodiesel, ethanol, methanol, 

compressed natural gas, and liquefied natural gas. 

59 Total GHG estimates for buses and the breakdown of emissions into these subcategories are somewhat uncertain, given 

different estimates of bus fuel consumption from different sources and limited data on fuel consumption by schoolbuses. For 

instance, estimates of fuel consumption by transit buses from the American Public Transportation Association (APTA) and fuel 

consumption from schoolbuses and intercity buses from the Eno Transportation Foundation systematically exceed the estimates 

of total bus fuel consumption reported by FHWA for the years 1990 to 2003. The estimates in this report and the U.S. GHG 

inventory rely on the FHWA data, and thus the GHG estimates reported here are lower than estimates that would result using the 

APTA and Eno data directly. 

60 Includes trolley buses. 

61 No data available for 2003 at time pf publication. U.S. Department of Transportation, Bureau of Transportation Statistics, 

2005. National Transportation Statistics 2004. Washington, DC, Table 1-32. 

62 Federal Highway Administration, 2004. Highway Statistics 2003. Washington, DC, Table MV-10. 

63 U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Table 5.13. 

20
Transportation GHG Emissions Report 

5 Aircraft 

5.1 Overview 

Aircraft produced about 9 percent of U.S. transportation greenhouse gas emissions in 2003 (173.1 Tg CO2 

Eq.) and were the largest source of non-road transportation GHGs. In total, aircraft GHG emissions 

decreased approximately 3 percent from 1990 to 2003. GHGs from military aircraft declined by 40 

percent over the period, and commercial aircraft GHGs increased moderately. 

Commercial aircraft64 produced 72 percent of U.S. aircraft GHGs in 2003 (124.0 Tg CO2 Eq.), which was 

4.7 percent greater than in 1990. The moderate increase reflected a 20 percent rise in GHGs from 1990 to 

2001, followed by a substantial decline following the terrorist attacks of September 11, 2001, and a small 

increase in 2003. Passenger travel rose much more rapidly than the level of GHG emissions, due to a 

higher number of occupied seats per plane and improved aircraft fuel efficiency. Consequently, GHG 

emissions per passenger-mile decreased 24 percent from 1990 to 2003, the largest improvement of any 

transportation mode. 

The remainder of aircraft GHG emissions in 2003 came from military aircraft (12 percent), general 

aviation65 aircraft (7 percent), and “other”66 aircraft (10 percent). In the U.S. GHG Inventory, aircraft 

emissions are based on domestic travel only, and exclude international travel to and from U.S. cities.67 

Commercial and military aircraft rely almost exclusively on jet fuel, while about one-quarter of the fuel 

used for general aviation is aviation gasoline.68 GHG emissions from aircraft in 2003 were 99 percent 

CO2, about 1 percent N2O, and less than 1 percent CH4. 

5.2 Factors Affecting Aircraft Emissions 

Aircraft emissions have risen due to increased air travel activity by both passengers and freight, but this 

has been offset to a large degree by the increased efficiency of aircraft and their operations. Between 1990 

and 2003, passenger-miles traveled on certificated domestic services increased by 48 percent, from 345.9 

billion to 505.2 billion passenger-miles. (In comparison, light-duty vehicle passenger-miles increased 31 

percent over the same timeframe.) The increase in air travel would likely have been greater if not for the 

terrorist attacks of September 11, 2001. From 1990 to 2000, commercial aircraft passenger-miles 

increased by an average of 4.1 percent annually; passenger miles dropped by 6.6 percent between 2000 

and 2002, and then increased by 4.7 percent in 2003.69 

Although air cargo accounted for less than 1 percent of total United States freight ton-miles in 2002, 

aviation was the fastest growing mode of freight transportation. Air ton-miles increased 63 percent from 

1993 to 2002. The value of air freight shipments nearly doubled over the same period, increasing from 

$395 billion in 1993 to more than $770 billion in 2002, at which point it represented 7 percent of the total 

64 Represents any aircraft used in “common carriage.” These are generally certificated air carriers (aircraft holding a certificate 

issued by the Federal Aviation Administration to conduct scheduled and/or non-scheduled (charter) services) and may carry 

passengers and/or freight. 

65 These are non-certificated civil aviation operations, typically operated for personal or business use. The types of aircraft used 

in general aviation range from corporate multiengine jet aircraft piloted by professional crews to amateur-built single-engine 

acrobatic planes to balloons and dirigibles. 

66 The balance of aircraft emissions are from “other” aircraft, which may include other uses of jet fuel such as heating oil. 

67 GHGs associated with international travel are reported in the Inventory under bunker fuel estimates. 

68 In total, aviation gasoline makes up about 1 percent of total aircraft fuel use. 

69 U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2004. 

Washington, DC,Table 1-11. 

21
Transportation GHG Emissions Report 

goods transported domestically. (Freight truck ton-miles increased 24 percent from 1993 to 2002, with the 

value of their cargo increasing 45 percent.) Based on the energy used per ton-mile, aviation is the most 

energy intensive mode of freight haulage. In 2001, the energy required to move a ton-mile of air cargo 

was 7.5 times greater than heavy-duty trucks, over 17 times that of ships, and 83 times greater than rail.70 

Using an energy intensity metric based on the monetary value of goods moved (such as Btu per dollar 

value shipped), air cargo is closer to other modes. However, it is also important to note that almost all air 

cargo shipments begin and end their journey by truck, meaning that the growth in air freight has increased 

demand for truck and intermodal services.71 

The energy intensity of passenger air travel has declined substantially, in part because of increased 

occupancy of aircraft. The average passenger load factor (percent of available seats that are occupied) on 

certificated air carriers’ domestic operations increased from 60.4 percent in 1990 to 72.4 percent in 2002, 

continuing a longer term pattern of increasing passenger loads (Figure 5-1). As a result, aircraft passenger 

miles grew faster than aircraft miles traveled between 1990 and 2000 (49 percent versus 43 percent).72 

Figure 5-1. Aircraft Passenger Load Factor, 1970–2003 

80% 

71.0% 72.4% 

70% 

1970 1975 1980 1985 1990 1995 2000 2003 

65.4% 

60.7% 60.4% 

60% 54.6% 

58.0% 

48.9% 50% 

40% 

30% 

20% 

10% 

0% 

Source: U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2004. Washington, DC. Table 

4-21. 

The reduced energy intensity of commercial aviation also reflects improvements in aircraft fuel 

efficiency.73 For new production aircraft, the fuel economy improvements have averaged 1 to 2 percent 

per year since the 1950s.74 These developments have been market-driven, as airlines have improved 

airframe and propulsion technology in order to reduce fuel costs. 

70U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, 2004. Transportation Sector Trend Data 

http://intensityindicators.pnl.gov/data.html . 

71 For more discussion, see: Bureau of Transportation Statistics, 2004. Freight Shipments in America: Preliminary Highlights 

from the Commodity Flow Survey Plus Additional Data. 

72 Aircraft miles traveled increased from 3.96 billion to 5.66 billion by certified carriers. Source: Bureau of Transportation 

Statistics, 2005. National Transportation Statistics 2004. Table 1-32. 

73 Measured in fuel consumed per aircraft-mile traveled. 

74 Intergovernmental Panel on Climate Change, September 1999. Aviation and the Global Atmosphere. Load Factor 

22
Transportation GHG Emissions Report 

One measure of fuel efficiency is the number of aircraft seat-miles per gallon of fuel consumed. The 

measure, aircraft seat-miles, is calculated by multiplying the total air mileage traveled by the total number 

of seats available.75 Available aircraft seat-miles per gallon increased by about 15 percent between 1990 

and 2003 (from 46 to 53 seat-miles per gallon), although about half of this gain occurred since 2001 as 

airlines reduced the number of flights. Nevertheless, the overall increases indicate the impact of longerterm improvements in aircraft fuel efficiency, as well as the retirement of older, less fuel-efficient aircraft 

(Figure 5-2).76 

Figure 5-2. Average Seat-Miles Traveled Per Gallon of Fuel Consumed, 1970–2003 

60

50

40

30

20

10

0 

1970 1975 1980 1985 1990 1995 2000 

Source: For years 1970, 1975, 1980, 1985, and 1990 and beyond, U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. 

National Transportation Statistics 2005. Washington, DC, Table 4-21. All other years estimated using linear regression. 

5.3 Other Considerations in Estimating Global Warming Impact 

The total effect of aircraft GHGs on global warming is difficult to discern and may not be entirely 

accounted for by examining the GWP of CO2, N2O, and CH4. In addition to the primary GHGs, jet 

engines produce NOX, which has two contrary effects. In the upper atmosphere, NOX leads to the 

production of ozone, which traps heat, but also indirectly destroys CH4. Aircraft also create condensation 

trails (contrails) under certain atmospheric conditions, which might have a greenhouse effect by allowing 

most of the solar radiation to pass and by absorbing infrared radiation from the Earth. These 

considerations lead to greater uncertainty in estimating the impact of air travel on global warming. Seat-Miles Traveled Per Gallon 

75 Vacant and occupied seats are considered equal.

76 Between 1990 and 2003, the amount of energy consumed per passenger-mile on commercial aircraft dropped by 30 percent, 

from 4,900 Btu’s per passenger-mile to about 3,500 Btu’s per passenger-mile. 

23
Transportation GHG Emissions Report 

6 Other Non-Road Transportation Sources 

6.1 Boats/Ships 

Boats and other marine vessels produced 3 percent of U.S. transportation GHG emissions in 2003 (58.0 

Tg CO2 Eq.), a 17 percent increase from 1990. However, these figures reflect wide fluctuations in year-toyear estimates of residual fuel consumption and are subject to a high degree of uncertainty. Much of this 

uncertainty results from the challenge of separating the domestic and international components of fuel 

consumption estimates. According to the UNFCCC reporting guidelines, national totals of GHG 

emissions should reflect only domestic transport, including the domestic leg of shipments bound for 

foreign markets. (The international component is represented by bunker fuel estimates). However, 

differentiating domestic and international fuel consumption is often difficult, resulting in significant yearto-year variations in the official estimates. 

Domestic water shipments77 declined by 27.3 percent from 1990 to 2003 (Figure 6-1). As a share of total 

freight movement, waterborne haulage declined from 24 percent of domestic ton-miles in 1993 to 16 

percent in 2002.78 Part of the decrease reflected shifts to trucks for goods movement, especially in the 

transport of lighter weight, time-sensitive commodities. Water shipments were also impacted by 

maintenance needs on the lock-and-dam systems, environmental constraints for river channel dredging 

and dam-controlled water levels, and a reduction in crude oil shipment from Alaska.79 In contrast to 

domestic water shipments, tons of waterborne imports and exports increased between 1990 and 2003, as 

the nation’s international trade grew. 

77 Measured in ton-miles. Waterborne commerce is comprised of several major elements: tugs and barges on the major rivers and 

inland waterways, oil tankers carrying Alaskan crude to California, ocean-going ships carrying various cargo between the 

continental United States, Hawaii, Alaska, and Puerto Rico, and large bulk vessels carrying coal, grain, and iron ore on the Great 

Lakes. 

78 U.S. Department of Commerce, U.S. Census Bureau, 2002 Economic Census, Transportation 2002 Commodity Flow Survey. 

Washington, DC. 

79 U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2004. 

Washington, DC, Table 1-52. 

24
Transportation GHG Emissions Report 

Figure 6-1. Ton-Miles Shipped by Domestic Water Transportation, 1990–2003 Ton-Miles Shippe

d

(millions) 

1,000,000 

1990 1992 1994 1996 1998 2000 2002 

800,000 

600,000 

400,000 

200,000 

0 

Source: U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2005. Washington, DC, Table 

1-46. 

Recreational boats make up about one-fifth of boat and ship GHG emissions. Recreational boat GHGs 

have increased steadily, growing from about 9.5 Tg CO2 Eq. in 1990 to 11.2 Tg CO2 Eq. in 2003, most 

likely the result of increased activity and a growing number of crafts. Between 1990 and 2002, the 

number of recreational boats registered in the United States increased by 17 percent from nearly 11.0 

million to 12.9 million.80 

6.2 Rail 

Rail produced 2 percent of total transportation GHG emissions in 2003, or 43.2 Tg CO2 Eq. This was an 

increase of 18 percent from 1990. About 89 percent of rail GHGs were from freight haulage, with the 

remainder coming from passenger sources such as urban transit rail,81 commuter rail, and intercity rail 

(Amtrak).82 GHG emissions from both freight and passenger rail increased. The majority of rail GHG 

emissions are from the combustion of diesel fuel (92 percent), with electricity use comprising the balance. 

6.2.1 Freight Rail 

Between 1990 and 2003, total ton-miles shipped via rail increased by 50 percent, an average annual 

growth rate of about 3.2 percent.83 Several factors contributed to the growth in ton-miles of rail 

shipments, including the economic expansion through much of the 1990s and steady growth in coal 

shipments. Demand also has grown for other bulk commodities that rely heavily on rail transport, 

including chemicals, lumber and wood products, and farm products. 

80 U.S. Department of Energy, 2004. Transportation Energy Data Book, Edition 24. Table 9.7. 

81 Especially the electricity used for heavy- and light-rail systems. 

82 The Energy Information Administration’s estimates of transportation electricity consumption, which are used to calculate CO2 

emissions for the U.S. GHG inventory, only account for electricity used by urban transit rail, not for intercity rail (Amtrak). As a 

result, the GHG electricity figures for rail are likely underestimated. Moreover, industry estimates of diesel fuel consumption by 

railroads exceeds the estimate calculated for the U.S. GHG Inventory, based on the apportionment of diesel fuel consumption to 

individual transportation sources (see Chapter 9 for a discussion of uncertainty). 

83 U.S. Department of Transportation, Bureau of Transportation Statistics, 2005. National Transportation Statistics 2003. 

Washington, DC, Table 1-46. 

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Transportation GHG Emissions Report 

Rail fuel economy has improved steadily from 1990 to 2003.84 Calculations by the Association of 

American Railroads (AAR) show that revenue ton-miles per gallon for Class I railroads85 has been 

increasing at a rate of about 1.6 percent annually over the last 12 years. In 2003, this measure was 405 

revenue ton-miles per gallon, up from 332 in 1990.86 This increase can be attributed to a number of 

factors, including the introduction of more efficient locomotives and lighter weight railcars. Railroads 

have also taken steps to improve the efficiency of their operations, by minimizing the movement of empty 

railcars and short trains. For example, railroads in the Great Plains states have closed many of their 

smaller spur lines and now carry grain shipments on 100-car “unit trains” that operate only on the 

railroads’ major trunk lines. 

6.2.2 Passenger Rail 

The increase in GHG emissions from passenger rail reflects a significant growth in passenger rail 

services, with a number of light-rail and commuter rail lines coming into service or expanding operations. 

Between 1990 to 2003, the number of vehicle miles of rail transit operations87 increased by 21.6 percent, 

from 560.9 million to 682 million vehicle miles. Meanwhile, commuter rail operations increased by 33 

percent, from 212.7 to 284 million vehicle miles. Amtrak train-miles and energy use also increased, 

although overall ridership declined from 1990 to 2003. Meanwhile, passenger-miles traveled on urban 

transit and commuter rail increased at an even higher rate than vehicle miles and fuel consumption. 

6.3 Pipelines 

While pipelines are not technically “mobile,” they are generally classified as part of the transportation 

sector because they serve an important purpose in transporting energy products domestically. (The U.S. 

GHG Inventory includes pipelines in its transportation sector estimates, but excludes them from the 

mobile source section.) More than 1.4 million miles of pipelines are used to transport natural gas, and 

almost 177,000 miles are used to transport crude oil and petroleum products in the United States. While 

there are far fewer miles of pipelines dedicated to transporting petroleum products, they account for twothirds of domestic petroleum transport. Pipelines also are used to transport coal slurry and water. 

According to the Commodity Flow Survey, pipelines accounted for about 17 percent of total ton-miles of 

all raw and finished products transported in 2002, similar to their share in 1993.88 

The two primary “fuels” used to operate pipelines are natural gas and electricity, which are used in 

pumps, motors, engines, and compressors. Natural gas used to power pipelines produced an estimated 35 

Tg CO2 in 2003, or about 2 percent of total GHG emissions from transportation, a figure that has stayed 

fairly constant since 1990.89 

84 Locomotive fuel efficiency is generally reported in ton-miles per gallon. Gross ton-miles are based on the movement of the 

entire train, including locomotives, railcars, and freight. Revenue ton-miles are based on the movement of freight for which the 

railroad collects revenue—roughly half of gross ton-miles. 

85 Class I railroads have operating revenues of more than $50 million. 

86 Association of American Railroads, 2004. Railroad Facts. Washington, DC, p. 40. 

87 Includes heavy and light rail. 

88 U.S. Department of Transportation, Bureau of Transportation Statistics, 2004. Freight Shipments in America: Preliminary 

Highlights from the Commodity Flow Survey Plus Additional Data. 

89 The reported estimates do not include CO2 emissions associated with electricity use, although electricity is a major power 

source for pipelines. According to estimates in the Transportation Energy Data Book, electricity used to power pipelines 

consumed about 3.0 billion kilowatthours (kWh) in 2002 (most recent year available). At the average rate of CO2 emitted per 

kWh, based on fuel mix in the United States, this level of electricity use equates to approximately 1.8 Tg CO2. 

26
Transportation GHG Emissions Report 

7 HFCs from Mobile Air Conditioners and Refrigerated Transport 

Hydrofluorocarbon (HFC) emissions from mobile air conditioners and transport refrigeration produced 

about 2 percent of total transportation GHG emissions in 2003. HFCs are used as a replacement for 

chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFCs), which deplete ozone and are 

required to be phased out through international agreement under the Montreal Protocol. (Official GHG 

Inventory estimates do not include CFC and HCFC estimates in national totals because of their mandated 

retirement.) As HFCs have replaced CFCs and HCFCs, their emissions have risen steadily from nearly 

zero emissions in 1990 to approximately 42.7 Tg CO2 Eq. in 2003 (Figure 7-1). While estimating the net 

effect of this transition is difficult, the introduction of HFCs has lowered GWP-weighted emissions 

because the replacement gases generally have lower GWPs than their predecessors. For instance, the 

standard automobile air conditioner refrigerant, HFC-134a, has a GWP of 1,300, compared with its 

predecessor’s net effect of between 7,300 and 9,900.90 

Figure 7-1. HFC Emissions from Mobile Air Conditioners and Refrigerated Transport, 1990-2003 Tg CO2 Eq. 

50 

40 

30 

20 

10 

0 

1990 1992 1994 1996 1998 2000 2002 

Source: U.S. Environmental Protection Agency, 2005. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. Washington, DC. 

Approximately 71 percent of HFC emissions in 2003 were associated with mobile air conditioners, which 

are used to cool the passenger compartments of on-road vehicles. The remaining HFC emissions came 

from refrigerated transport. HFCs are commonly used to refrigerate perishable food and temperaturesensitive items during transport on reefer ships, refrigerated freight trains, and insulated trucks and trailers 

with self-contained refrigeration units. 

HFC-134a was introduced in some automobiles beginning in 1992 and became the standard automobile 

refrigerant in 1994. Some vehicles with CFC air conditioners were retrofitted with HFC-134a or a 

refrigerant blend. Regulations and industry practices established during the CFC phase-out, such as better 

training of technicians, certification requirements to purchase CFC refrigerant, and requirements to use 

recovery equipment and not vent refrigerant during equipment service, have reduced leakage and other 

“unnecessary” emissions. Some of these practices and requirements have also been implemented with 

newer HFC-equipped vehicles, which has helped limit the growth of HFC emissions. Manufacturers have 

also become increasingly sensitive to the environmental effects of refrigerants. Many have responded by 

reducing refrigerant charge sizes and leak rates, increasing reliability of their equipment, and 

investigating refrigerants with even lower GWPs. 

90 CFC-12 was the predecessor to HFC-134a. Although CFC-12 has an estimated direct GWP of 10,600, it also destroys 

stratospheric ozone, thus lowering its net effect. 

27
Transportation GHG Emissions Report 

Other vehicle air conditioning systems also are transitioning away from ozone depleting chemicals.91 

Most buses and many passenger trains currently use HCFC-22, which is an ozone depleting chemical that 

will be phased out under the Montreal Protocol. HCFC-22 has an ozone depleting impact estimated to be 

5.5 percent that of CFC-12. While production is currently allowed under the Montreal Protocol in all 

countries, many nations have accelerated phase-out through regulations and other measures. Some new 

buses and trains have utilized alternatives to HCFC-22, most notably HFC-134a, R-407C (a blend of 

HFC-32, HFC-125, and HFC-134a), and R-410A (a blend of HFC-32 and HFC-125). 

CFC and HCFC emissions are not included in official U.S. GHG inventory estimates. Due to the required 

phase-out of these gases, their emissions have been steadily declining (Figure 7-2). The use of HFCs as a 

replacement to CFCs and HCFCs has resulted in lower GWP-weighted emissions, although there is no 

official estimate of the net change in GWP-weighted emissions. 

Figure 7-2. CFC and HCFC Emissions from Mobile Air Conditioners and Refrigerated Transport, 

1990–2003 

100 

80 

60 

40 

20 

0 

1990 1992 1994 1996 1998 2000 2002 

NOTE: CFC and HCFC emissions are not included in the Inventory of U.S. Greenhouse Gas Emissions and Sinks. 

Source: U.S. Environmental Protection Agency, 2004. U.S. EPA Vintaging Model. Version VM IO file_11-08-04. Metric Tons (thousands) 

91 In the past, transportation-related air conditioning and refrigeration systems have used a wide variety of CFCs and HCFCs. 

Most notable have been CFC-12, HCFC-22, and R-502 (a blend of CFC-115 and HCFC-22). These are being replaced primarily 

by HFCs, although some non-fluorocarbon alternatives such as ammonia, hydrocarbons, and water also are used. CFC-12 and R502 have been replaced by HFC-134a, R-404A (a blend of HFC-125, HFC-143a, and HFC-134a) and R-507A (a blend of HFC125 and HFC-143a). HCFC-22 is still used but also is being replaced by these refrigerants as well as R-407C (a blend of HFC-32, 

HFC-125 and HFC-134a) and R-410A (a blend of HFC-32 and HFC-125). 

28
Transportation GHG Emissions Report 

8 Non-Transportation Mobile Sources—Agricultural and 

Construction Equipment, Recreational Vehicles, and Other 

This report focuses on GHG emissions from the transportation sector. For the most part, transportation 

sources are associated with the movement of people and goods. There are several other mobile sources 

that serve functions other than transportation, such as construction or shelter. These “non-transportation 

mobile” sources were estimated to have produced 144.8 Tg CO2 Eq. in 2003.92 

Non-transportation mobile sources include: 

• Agricultural equipment—This category predominantly consists of tractors, mowers, 

combines, balers, and other farm-related equipment that perform functions while moving. 

There are twice as many diesel vehicles as gasoline vehicles in this category. 

• Construction equipment—Construction equipment includes cement and mortar mixers, 

excavators, forklifts, loaders, bore drill rigs, and other equipment. 

• Recreational vehicles—These vehicles include snowmobiles, off-road motorcycles, all-terrain 

vehicles, and golf carts. 

• Lawn and garden equipment—This equipment type includes lawnmowers, lawn and garden 

tractors, snowblowers, leaf blowers, and other equipment used for residential and commercial 

purposes. 

• Other commercial and industrial equipment—This source includes equipment such as airport 

ground service equipment, aerial lifts, sweepers/scrubbers, and other utility equipment. 

Figure 8-1. Greenhouse Gas Emissions from Non-Transportation Mobile Sources, 1990 and 2003 

39.0 

30.5 

12.1 

6.6 

9.8 

2.6 

59.1 

42.3 

14.3 

8.9 

17.4 

2.8 

0 

10 

20 

30 

40 

50 

60 

Tg CO2 Eq. 

1990 

2003 

Construction Farm Lawn and Recreational Ind/ Other* 

Equip. Equip. Garden Equip. Comm. 

Equip. Equip. 

Source: Calculated by summing estimates of N2O, CH4, and CO2. N2O and CH4 estimates were taken from U.S. Environmental Protection Agency. 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. Washington, DC, Tables 3-20 and 3-21. CO2 estimates were developed based 

92 The U.S. GHG Inventory presents estimates of CH4 and N2O from non-transportation mobile sources but does not provide 

estimates of CO2 emissions, since fuel consumption from these sources is not broken out as a separate data element in the Energy 

Information Administration’s fuel data. In the U.S. GHG Inventory, CO2 from these sources is encompassed in other sectors 

(e.g., industrial) and is not included among transportation sources. As a result, this report includes CO2 estimates calculated for 

these non-transportation mobile sources based on fuel consumption estimates from EPA’s NONROAD Model, which are used in 

the CH4 and N2O inventory calculations for mobile sources. 

29
Transportation GHG Emissions Report 

on fuel consumption data contained in U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2003. 

Washington, DC, Annex 3, Table 3-13. 

Although there is considerable uncertainty93 associated with these estimates, it appears likely that GHG 

emissions from non-transportation mobile sources were nearly equal to the combined GHGs of boats and 

ships, rail, and pipelines in 2003. Collectively, GHG emissions from non-transportation mobile sources 

increased by 44 percent from 1990 (Figure 8-1). CO2 accounted for more than 99 percent of GHG 

emissions, and N2O and CH4 each produced less than 1 percent. 

93 Activity and fuel consumption data for these sources are limited in comparison to transportation sources. There is no one data 

source that currently has information on all the non-transportation mobile sources, and different publications report significantly 

different estimates, complicating the estimation process. For instance, estimates derived from a 2004 analysis of various data 

sources, including FHWA, EPA, and EIA by Oak Ridge National Laboratory (ORNL), were considerably higher than estimates 

currently used in developing the U.S. GHG Inventory. Moreover, it is likely that the transportation and mobile sources estimates 

of GHG emissions in the U.S. GHG Inventory are missing emissions associated with off-road use of trucks, and these emissions 

may be captured under other sectors, especially the industrial sector. 

30
Transportation GHG Emissions Report 

9 Estimating Transportation GHG Emissions—Methodology and 

Uncertainty 

All GHG emissions estimates presented in this report are from the official Inventory of U.S. Greenhouse 

Gas Emissions and Sinks, published annually by EPA.94 These estimates are calculated using data such as 

vehicle miles traveled, fuel consumption, and emissions factors. The quality and reliability of these data 

sources significantly determine the reliability of GHG emissions estimates. This chapter briefly describes 

the methods used to calculate transportation GHG emissions, and then discusses the uncertainty 

associated with these methods and the supporting data sources. 

9.1 Carbon Dioxide Emissions 

9.1.1 Methodology 

Carbon dioxide emissions are a direct product of fossil fuel combustion. They are calculated for each fuel 

type as a simple product of the following factors: 

• Fuel consumption (in Btus) – These estimates are based on data provided by EIA. 

• The carbon coefficient of a particular fuel – These values represent the total amount of carbon 

released when the fuel is burned (expressed in Tg carbon per Btu). The carbon coefficient 

depends on the density, carbon content, and gross heat combustion of the fuel. 

• The percent of fuel that is combusted – The 1996 IPCC assumes that oxidation is 99 percent 

complete and that 1 percent of the carbon remains sequestered.95 

In the U.S. GHG Inventory, CO2 emissions from the transportation sector are estimated using a multistage 

process. First, total national CO2 emissions from fossil fuel combustion are calculated by accounting for 

the factors described above. National-level estimates of fuel consumption by fuel type are multiplied by 

the carbon content of each fuel and the percent of fuel that is oxidized, producing fuel-specific CO2 

estimates. These fuel-specific CO2 estimates then are apportioned to economic sectors based on each 

sector’s contribution to total fuel consumption. Within each sector, emissions then are allocated to 

individual sources (such as particular transportation modes) based on fuel consumption data. 

9.1.2 Uncertainty 

Since the vast majority of transportation GHG emissions are in the form of CO2, uncertainty in the CO2 

estimates has a much greater effect on the transportation sector estimates than uncertainty associated with 

N2O, CH4, or HFC emissions. EPA believes that the uncertainty in CO2 estimates for the United States as 

a whole is relatively small. 

As described in Chapter 3 of the U.S. GHG Inventory, the uncertainty associated with total CO2 

emissions from fossil fuel combustion is a function of the uncertainty in primary input data: fuel 

consumption, carbon content, and oxidation factors. Fuel-specific consumption data are obtained from 

EIA, primarily from its Monthly Energy Review, as well as unpublished sources within the agency. EIA 

also provides the carbon contents. Oxidation fractions are published by the IPCC. 

94 Non-transportation mobile estimates for CO2 are not provided in the body of the Inventory report but are calculated in the 

Inventory Annex. For this report, a minor correction was made to the diesel CO2 figures for non-transportation mobile sources, so 

the figures reported differ slightly from those in the Inventory Annex. Some emissions from sources considered “nontransportation mobile sources” in this report are counted as part of transportation sector emissions in the Inventory. 

95 Note that the 2006 IPCC guidelines specify a 100 percent oxidation fraction. 

31
Transportation GHG Emissions Report 

Fuel sales are tracked for many reasons, such as taxation and economic analyses. On an aggregate level, 

fuel consumption data are believed to be relatively accurate. The carbon content of fuels and oxidation 

fraction values are also relatively certain. As a result, the total estimate of CO2 emissions from fossil fuel 

combustion reflects a small degree of presumed uncertainty, estimated to range from 1 percent below the 

U.S. GHG Inventory’s actual estimate to 6 percent above it, based on a 95 percent confidence interval.96 

However, additional uncertainty is introduced when the national totals are allocated to individual sectors 

(such as transportation) and sources within sectors (such as modes and vehicle types). The allocation 

process is based on the relative consumption of fuel by each individual source. For example, if 

transportation is estimated by EIA to comprise X percent of national gasoline consumption, then the 

transportation sector is allocated X percent of CO2 emissions from gasoline. The CO2 emissions then are 

allocated to individual modes and vehicle types by EPA based on fuel consumption data from a variety of 

sources, including FHWA, American Public Transportation Association, AAR, and DOE. 

The apportionment methodology used to develop CO2 estimates for the GHG Inventory represents a “topdown” calculation approach. These values are somewhat different than estimates that would be calculated 

“bottom-up” starting with primary data sources, such as FHWA’s Highway Statistics. These differences 

are a source of uncertainty in the transportation GHG estimates. 

Differences in fuel consumption reported by EIA and other sources largely stem from different survey 

methodologies, data collection processes, and allocations of fuel use. EIA’s estimates of transportation 

diesel fuel consumption, which are used in the official GHG inventory, are systematically 2.5 to 10.0 

percent lower than estimates from various bottom-up sources for 1990 to 2003.97 EIA’s estimates of 

transportation motor gasoline fuel consumption for 1990 to 2003 also are systematically lower by a small 

amount (ranging from 0.6 to 2.4 percent) than estimates compiled by EPA using FHWA’s Highway 

Statistics for on-road vehicles and EPA’s NONROAD Model for recreational boats. On the other hand, 

EIA’s estimates of transportation jet fuel use are consistently higher (9.1 to 12.3 percent) than estimates 

compiled by EPA for 1990 to 2003. 

Using the “bottom-up” method described above, VMT were apportioned by fuel type and then allocated 

to individual model years using temporal profiles of both the vehicle fleet by age and vehicle usage by 

model year in the United States provided by EPA (2004b) and EPA (2000). Although the uncertainty 

associated with total U.S. VMT is believed to be low, the uncertainty within individual source categories 

was assumed to be higher, given uncertainties associated with apportioning total VMT into individual 

vehicle categories, by technology type, and equipment age. The uncertainty of individual estimates was 

assumed to relate to the magnitude of estimated VMT (i.e., it was assumed smaller sources had a greater 

percentage of uncertainty). A further source of uncertainty occurs because FHWA and EPA use different 

definitions of vehicle type, and estimates of VMT by vehicle type (provided by FHWA) are broken down 

by fuel type using EPA vehicle categories. The bottom-up estimates are also subject to several possible 

sources of error, such as unregistered vehicles, unreported fuel sales to avoid fuel taxes, differences in 

achieved versus estimated fuel economy, and measurement and estimation errors. 

Despite these issues, EPA believes that the uncertainty associated with CO2 from the transportation sector 

is still relatively small. Depending on the fuel type, these values range from 6 percent below to 7 percent 

above actual estimates based on a 95 percent confidence interval. However, it is likely that the uncertainty 

for individual modes is higher. For instance, FHWA is recognized as being the preeminent data source for 

96 Based on Monte Carlo simulations, which randomly generate values for uncertain variables repeatedly. These randomly 

generated numbers are used by the simulation to estimate results. For a 95 percent confidence interval, estimated values fell 

within the specified range for 19 out of 20 simulations. 

97 These include FHWA’s Highway Statistics for highway vehicles, EIA’s Fuel Oil and Kerosene Sales Report for ships and 

boats, and individual data sources for locomotives (AAR for Class I railroads, the Upper Great Plains Transportation Institute for 

Class II and III railroads, and the Transportation Energy Data Book for commuter rail and Amtrak). 

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Transportation GHG Emissions Report 

on-road vehicle fuel use, and EIA’s transportation sector totals are estimated using FHWA data for onroad vehicles and other survey data for non-road transportation sources. As a result, uncertainties in nonroad fuel use are believed to be largely responsible for discrepancies in consumption of motor gasoline 

and diesel fuel between EIA and other sources. Nonetheless, if CO2 estimates from EIA are lower than 

estimates that would be developed using bottom-up data sources, CO2 emissions for all modes are 

reduced proportionately using the current allocation method. 

Table 9-1 provides a summary comparison of estimates of transportation CO2 emissions reported in the 

U.S. GHG Inventory for 1990 and 2003 compared with estimates developed based on a bottom-up 

approach. 

Table 9-1. Comparison of U.S. GHG Inventory Estimates and Bottom-Up Estimates of CO2 for 

Selected Transportation Fuels and Sources 

 1990 2003 

Fuel Type/Vehicle Type 

Inventory 

Est. 

Bottom-Up 

Est. 

Difference Inventory 

Est. 

Bottom-Up 

Est. 

Difference 

Gasoline 

Automobiles 

Light-Duty Trucks 

Heavy-Duty Trucks 

Buses 

Motorcycles 

Boats (Recreational) 

955.2 

605.1 

301.0 

37.7 

0.3 

1.7 

9.4 

973.5 

616.7 

306.7 

38.5 

0.3 

1.7 

9.6 

18.3 

11.6 

5.7 

0.8 

0.0 

0.0 

0.2 

1,143.7 

630.2 

460.9 

39.6 

0.3 

1.6 

11.0 

1,153.9 

635.8 

465.0 

39.9 

0.3 

1.6 

11.1 

10.2 

5.6 

4.1 

0.3 

0.0 

0.0 

0.1 

Diesel Fuel 

Automobiles 

Light-Duty Trucks 

Heavy-Duty Trucks 

Buses 

Locomotives 

Ships and Boats 

253.7 

7.4 

10.7 

178.4 

7.5 

33.3 

16.3 

264.1 

7.7 

11.2 

186.4 

7.8 

34.8 

16.2 

10.4 

0.3 

0.5 

8.0 

0.3 

1.5 

-0.1 

386.6 

3.4 

17.6 

301.1 

8.0 

39.6 

17.0 

417.0 

3.7 

19.0 

325.5 

8.6 

42.8 

17.4 

30.4 

0.3 

1.4 

24.4 

0.6 

3.2 

0.4 

Electricity 3.0 3.2 0.2 3.2 3.9 0.7 

Jet Fuel 

Commercial Aircraft 

Military Aircraft 

General Aviation Aircraft 

Other Aircraft 

174.2 

117.2 

34.8 

6.3 

15.9 

158.2 

117.2 

34.8 

6.3 

-

-16.0 

0.0 

0.0 

0.0 

169.0 

122.8 

20.5 

9.4 

16.3 

152.7 

122.8 

20.5 

9.4 

-

-16.3 

0.0 

0.0 

0.0 

As shown in this table, bottom-up estimates suggest that GHG emissions from transportation diesel fuel 

use may be higher than reported in the U.S. GHG Inventory, particularly from diesel fuel used by heavyduty trucks. GHG emissions from motor gasoline also may be higher, although the percentage difference 

in these estimates is smaller than those of diesel. Electricity fuel consumption in transportation also 

appears higher using a bottom-up methodology and shows a higher rate of growth; much of this 

difference is likely attributable to increased electrification of Amtrak service in the Northeast Corridor, 

which is not reflected in the EIA estimates. The total GHG estimates presented in the U.S. GHG 

Inventory are believed to account for all these emissions, but the transportation sector estimate may not. 

Finally, a recent EPA study suggested that the fraction of fuel combusted for light-duty gasoline motor 

vehicles is 100 percent. The revised estimate has been peer reviewed and may be incorporated into future 

IPCC guidance. It also is possible that diesel and gasoline vehicles burn virtually 100 percent of their fuel, 

and EPA will be conducting further research to examine these estimates for transportation and nontransportation sources. 

33
Transportation GHG Emissions Report 

9.2 Methane and Nitrous Oxide Emissions 

9.2.1 Highway Vehicles 

Unlike CO2 emissions, which are directly proportional to fuel consumption, CH4 and N2O emissions from 

highway vehicles are affected by vehicle emissions control technologies. Emissions are calculated based 

on VMT and per-mile emissions factors, which vary by type of emissions control technology system. The 

total VMT driven within each class of vehicles is distributed among various emissions control systems, 

based on distributions of vehicles by model year, VMT by vehicle age, and control technologies in place 

by model year. 

Uncertainty in CH4 and N2O emissions from highway vehicles is a product of uncertainty in VMT 

estimates, the distribution of VMT to control technology types, and emissions factors. 

VMT estimates by vehicle type are taken from FHWA. These estimates are believed to be relatively 

accurate at the national level but are subject to several possible sources of error. The VMT are 

apportioned by fuel type and then allocated to individual model years using EPA temporal profiles of both 

the vehicle fleet by age and vehicle usage by model year in the United States. Although the uncertainty 

associated with total U.S. VMT is believed to be low, the uncertainty within individual vehicle categories 

is assumed to be higher, given uncertainties associated with apportioning total VMT into individual 

vehicle categories, by technology type, and equipment age. 

The emissions factors for highway vehicles used in the U.S. GHG Inventory are based on laboratory 

testing of vehicles. Although the controlled testing environment simulates actual driving conditions, the 

results from such testing can only approximate real world conditions and emissions. For some vehicle and 

control technology types the testing did not yield statistically significant results within the 95 percent 

confidence interval, requiring reliance on expert judgment when developing the emissions factors. In 

those cases, the emissions factors were developed based on comparisons of fuel consumption between 

similar vehicle and control technology categories. 

The U.S. GHG Inventory reports that the uncertainty range of CH4 and N2O emissions is greater than that 

of CO2 emissions. CH4 is estimated to be somewhere between 9 percent below and 4 percent above the 

actual U.S. GHG Inventory total, based on a 95 percent confidence interval. The calculated value of N2O 

emissions has even greater uncertainty, with uncertainty estimates ranging from 16 percent below to 26 

percent above the U.S. GHG Inventory total, based on a 95 percent interval. However, the overall 

significance of uncertainty in CH4 and N2O estimates is presumed to be relatively minor because these 

emissions comprise a small portion of total highway vehicle GHG emissions. 

9.2.2 Other Mobile Sources 

The U.S. GHG Inventory calculates CH4 and N2O emissions for other mobile sources by applying an 

emissions factor to the quantity of fuel consumed. The uncertainty of these calculations is a direct product 

of uncertainties in these two inputs, which often are considered to be highly uncertain. For example, the 

IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories 

reports that CH4 emissions from aviation and marine sources may be uncertain by a factor of two, while 

N2O emissions may be uncertain by an order of magnitude for marine sources and several orders of 

magnitude for aviation. No information is provided on the uncertainty of emissions factors for other nonroad sources. 

Fuel consumption data are drawn from a variety of sources. Consumption tracking for some modes, 

particularly for the less significant modes, is not highly accurate. Fuel consumption for some modes is 

estimated using EPA’s NONROAD model and is not based on actual sales records for each year. Sales 

data often cannot accurately reflect the actual end use of a given fuel. For instance, some gasoline 

purchased by the marine sector may be used for operating heavy equipment or even generators, instead of 

34
Transportation GHG Emissions Report 

being used entirely by ships and boats. This distinction between mobile and stationary fuel users is not 

made by EIA. 

An even greater level of uncertainty is associated with the emissions factors themselves. The U.S. GHG 

Inventory relies on emissions factors provided by the IPCC. In some cases these factors cover very broad 

categories. For example, the same emissions factors are used for tractors, snowmobiles, riding 

lawnmowers, and construction vehicles. It is likely that these various modes emit differing amounts of 

non-CO2 gases per gallon of fuel consumed. As another example, a single emissions factor is applied to 

all jet fuel consumed, even though emissions amounts vary depending on whether the aircraft is in the 

landing/take-off cycle or the cruise portion of flight. 

Despite the large degree of uncertainty associated with non-road modes, the significance of this 

uncertainty is low given the relatively small quantity of GHGs released by these sources. 

35
Transportation GHG Emissions Report 

10 Lifecycle Transportation Emissions 

This report primarily addresses GHG emissions from energy that is used for powering vehicles. 

Transportation depends on array of additional processes, such as the manufacture of vehicles and 

extraction of crude oil. Within the U.S. GHG Inventory, these activities are accounted for in other 

economic sectors—most notably the industrial sector. Nevertheless, they are still a part of the 

transportation lifecycle and can offer a broader perspective on the GHG impact of transportation.98 

A full lifecycle assessment (LCA) of transportation takes into account all emissions associated with the 

vehicles, fuel, infrastructure, and associated activities that make up the nation’s transportation system. 

Emissions occur during three lifecycle stages: 

1. Upstream Emissions—Upstream emissions are those that occur before a product is used, 

including extraction of raw materials, processing, manufacturing, and assembly. Sources of 

upstream emissions include any fuel combustion associated with these processes, as well as 

“fugitive” emissions, such as venting and/or flaring of natural gas from oil wells or natural 

gas plants. 

2. Direct Emissions—Direct emissions occur during the operation and maintenance of vehicles. 

3. Downstream Emissions—Downstream emissions occur at the end of the lifecycle and are 

associated primarily with disposal. Sources of downstream emissions include fuel combustion 

used during disposal, collection of municipal solid waste, and landfills. 

An LCA of transportation also should take into account emissions from three key components of 

transportation systems: fuels, vehicles, and infrastructure. 

Table 10-1 provides examples of sources of emissions at each stage of life for each component. 

Transportation fuel use is the focus of traditional analysis of transportation emissions. An LCA of 

transportation fuels, often referred to as a fuel cycle analysis, includes upstream emissions associated with 

drilling, exploration and production, crude oil transport, refining, fuel transport, storage, and product 

retail, as well as downstream disposal or recycling of oil products. 

An analysis of vehicle lifecycle emissions includes each stage of vehicle manufacturing (raw material 

extraction, processing, and transport; manufacture of finished materials; assembly of parts and vehicles; 

and distribution to retail locations), vehicle operation and maintenance, and vehicle disposal. 

Finally, an LCA of infrastructure includes emissions associated with construction, operation and 

maintenance, and disposal of all transportation-related infrastructure, such as roads, parking lots, 

pipelines, railroad tracks, bridges, tolls, airports, train and bus stations, and fuel stations. 

A lifecycle assessment can be useful in evaluating certain policy questions. This approach is increasingly 

used in the transportation sector to compare emissions from different fuel types, especially when the 

emissions generated in fuel production may vary significantly from the tailpipe emissions during 

combustion. 

98 Although official estimates of national GHG emissions do not usually take a lifecycle approach, there are some exceptions. 

Fuel ethanol derived from biomass is assumed to have net CO2 emissions of zero, as crops sequester carbon from the atmosphere 

while they grow. Similarly, in some places in the U.S. Inventory, electricity emissions are accounted for in the transportation 

sector, although they are all upstream emissions. 

36
Transportation GHG Emissions Report 

Table 10-1. Elements of the Transportation Lifecycle 

Vehicle Cycle Fuel Cycle Infrastructure Cycle 

Upstream 

Emissions 

Upstream Vehicle Cycle 

Raw material (e.g., ore for steel 

or aluminum; petroleum for 

plastics) extraction, processing, 

production, and transport; 

manufacture of finished 

materials and components; 

intermediate parts 

transportation; assembly of 

parts and vehicles; distribution 

to retail locations 

Upstream Fuel Cycle 

Exploration, drilling, production, 

and pumping; agricultural 

activities for biomass; 

production activities for other 

energy sources; crude 

oil/gas/material transport; 

refining and processing into 

motor fuel; product transport, 

intermediate, wholesale, and 

retail storage; retail product 

sales and dispensing 

Upstream Infrastructure 

Cycle 

Raw material production and 

transport (e.g., asphalt, 

cement, and steel); 

desequestration (clearcutting) of land; construction 

activities 

Direct 

(Operating) 

Emissions 

Direct Vehicle Cycle 

Tire wear; engine oil and other 

lubricant and fluid use; parts 

replacement; other operations 

and maintenance activities 

Direct Fuel Cycle 

Fuel combustion; fuel 

evaporation [This element is 

the only one covered under 

traditional transportation 

emissions analyses.] 

Direct Infrastructure Cycle 

Resurfacing; repainting and 

striping; pothole repair; 

plowing, street cleaning, other 

operations and maintenance 

activities 

Downstream 

Emissions 

Downstream Vehicle Cycle 

Disposal of vehicles, including 

possible recycling of parts; tire 

disposal and possible 

incineration 

Downstream Fuel Cycle 

Disposal and possible recycling 

of oil products 

Downstream Infrastructure 

Cycle 

Disposal and possible 

recycling of certain 

infrastructure raw materials; 

potential reclamation of land 

(e.g., rails-to-trails) 

10.1 Estimates of Transportation-Related CO2 Emissions 

A lifecycle analysis of CO2 emissions from the nation’s transportation system was developed for this 

report, examining upstream fuel cycle and vehicle cycle emissions. This analysis did not account for 

emissions from the infrastructure cycle, although their potential impact is recognized to be potentially 

significant. Nevertheless, the estimates in this section offer an initial perspective on some of the 

additional GHG impacts associated with various vehicle types and modes, as well as the transportation 

sector as a whole. 

Special consideration needs to be given to potential areas of double-counting. For example, a 

comprehensive LCA of the GHG impacts of passenger vehicles includes emissions from the transport of 

crude oil and motor gasoline used by these vehicles, as well the original shipment of these vehicles from 

the automotive manufacturing plant to the dealer. However, upstream transport-related emissions are 

already represented in the direct emissions from other vehicle types, such as heavy-duty vehicles used to 

transport new passenger cars. While attributing these transport-related emissions to passenger cars is 

acceptable when examining the lifecycle impacts of these vehicles, it is not appropriate to include these 

when considering the sector as a whole. Therefore, all upstream and downstream transport-related 

emissions should be excluded when examining the entire sector. 

Two leading transportation lifecycle models were used to assess GHG impacts for this analysis. The 

Lifecycle Emissions Model (LEM) was developed by Mark A. Delucchi of the Institute of Transportation 

Studies at the University of California-Irvine. The Greenhouse Gases, Regulated Emissions, and Energy 

37
Transportation GHG Emissions Report 

Use in Transportation (GREET) model was developed by Argonne National Laboratory for DOE’s Office 

of Transportation Technologies. Each of these models is described briefly below.99 

10.1.1 Lifecycle Emissions Model (LEM) 

The Lifecycle Emissions Model100 examines energy use, GHG emissions (CO2, CH4, N2O, HFCs), and 

criteria pollutant emissions associated with the full lifecycle of various transportation activities. This 

model examines the following components: 

 Fuel cycle—raw material production (e.g., crude oil), raw material transport, fuel production, 

fuel distribution and storage, fuel dispensing, and end use; 

 Material lifecycle—raw material recovery (e.g., iron ore), vehicle manufacture, and transport 

of materials to end-users; 

 Vehicle lifecycle—assembly, operations and maintenance, secondary fuel cycle; and 

 Infrastructure lifecycle—energy use and materials production. 

Lifecycle emissions for a number of vehicle types are calculated, including passenger cars, buses, and 

medium- and heavy-duty trucks. No estimates regarding other vehicle types or any stage of infrastructure 

lifecycle emissions have been included, as those estimates in LEM are still considered rudimentary. 

10.1.2 GHGs, Regulated Emissions, and Energy Use in Transportation (GREET) 

GREET 1.6101 estimates energy use, GHG emissions (CO2, CH4, N2O), and criteria pollutant emissions 

related to the fuel cycle of various vehicle and fuel combinations. The primary purpose of GREET is to 

evaluate the energy and emissions impacts associated with alternative fueled vehicles and advanced 

vehicle technologies in light-duty vehicles, for the purpose of assessing near- and long-term transportation 

options. GREET examines more than 30 fuel-cycle pathways, and examines the following components: 

 Feedstock production; 

 Feedstock transportation; 

 Feedstock storage; 

 Fuel production, 

 Fuel transportation and distribution; 

 Fuel storage; and 

 Vehicle operation (refueling, fuel combustion/conversion, fuel evaporation, tire/break wear). 

10.1.3 Results 

Both GREET and LEM provide estimates of upstream fuel cycle and vehicle cycle emissions for various 

vehicle and fuel categories. While both models are capable of estimating emissions for alternative fuels, 

this analysis focused on the upstream and direct emissions from gas and diesel vehicles, since they 

comprise the majority of transportation emissions. For each lifecycle component, ranges of emissions for 

99 Although the LEM model does not specifically account for downstream emissions, the GREET model does include emissions 

resulting from the fuel cycle portion of the transportation lifecycle. 

100 Delucchi, M. 2003a. Lifecycle Emissions Model (LEM), Mark A. Delucchi, Institute of Transportation Studies, University of 

California, December. 

101 U.S. Department of Energy, Center for Transportation Research, 2001. Greenhouse Gases, Regulated Emissions, and Energy 

Use in Transportation (GREET) 1.6 Model. Argonne National Laboratory, June 2001. 

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Transportation GHG Emissions Report 

each vehicle/fuel category were developed based on minimum and maximum values from both models. 

Ranges were used to account for some of the assumptions and uncertainties behind these models. 

Upstream Fuel Lifecycle Emissions 

Elements of the upstream fuel cycle that were examined include extraction, shipment, refining, and 

distribution of raw materials and finished products. Although the GREET model uses these categories, the 

categories in LEM had to be mapped to this configuration. This mapping was performed using 

information provided by the developer of LEM.102 

From each model, CO2 emissions per million Btu (MMBtu) of fuel for each of these components were 

obtained by mode, vehicle, and fuel type. Ratios of upstream emissions to direct emissions then were 

determined for each component, as shown in Table 14-1. 

Upstream Vehicle Lifecycle Emissions 

Emissions from the upstream vehicle lifecycle were estimated only for highway vehicles. (Neither model 

has been used to evaluate non-highway vehicle cycle emissions.). Ratios of vehicle lifecycle emissions to 

direct fuel cycle emissions were obtained from LEM as inputs into this analysis. 

The LEM model was used to specify the ratio of vehicle cycle emissions to direct fuel cycle emissions for 

gasoline light-duty vehicles and for diesel heavy-duty. It was assumed that the vehicle cycle for gasoline 

light-duty vehicles could be used as a proxy for diesel light-duty vehicles and that the vehicle cycle for 

diesel heavy-duty vehicles could be used as a proxy for gasoline heavy-duty vehicles. This proportion of 

total upstream vehicle cycle emissions then was disaggregated to transport- and non-transport-related 

emissions. Ratios of upstream emissions to direct emissions also are shown in Table 14-1. 

Total Lifecycle Emissions 

To estimate total lifecycle emissions, emissions from the upstream fuel and vehicle cycles were summed 

for each mode, vehicle type, and fuel type. These total estimates in Table 14-1 represent the ratio of 

lifecycle emissions to direct emissions for each vehicle/fuel category. 

These total estimates are valid when assessing the CO2 impact over the lifecycle of each vehicle and fuel 

combination individually. However, it is important to recognize that some of the upstream emissions are 

currently represented in the transportation totals of the U.S. GHG Inventory. In the upstream fuel cycle, 

shipment and distribution of fuel fits in this category; in the upstream vehicle cycle, “transport” of 

vehicles fits in this category. These components were subtracted out of the proportion of total emissions 

to arrive at “total less transport.” For example, GHG emissions associated with the lifecycle of passenger 

cars running on conventional gasoline were estimated to be 1.35 to 1.43 times that of direct emissions, 

when taking out transportation-related emissions that are counted elsewhere in the U.S. GHG Inventory. 

The ratios of upstream fuel and vehicle cycle emissions shown in Table 14-1 then were applied to total 

U.S. CO2 emissions from direct fuel combustion for each vehicle/fuel type to estimate total lifecycle 

GHG emissions. These emissions are shown in Table 14-2. 

Based on these results, total lifecycle emissions for the nation’s transportation sector are estimated to be 

27 to 37 percent higher than direct fuel combustion emissions. These estimates do not include some 

important components of the transportation lifecycle, such as upstream vehicle cycle emissions for nonhighway vehicles, and emissions from the construction and maintenance of infrastructure. However, it 

should also be noted that these estimates do include upstream emissions that take place outside the United 

States, such as fuel produced and vehicles manufactured abroad that are used in the nation’s 

transportation system. As a result, the total GHG emissions presented in Table 14-2 reflect some 

102 Delucchi, M. 2003b. Personal Communication between Mark Delucchi of the University of California and Bill Cowart of 

ICF Consulting. June 14, 2003. 

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Transportation GHG Emissions Report 

emissions that are not included in the official U.S. GHG Inventory estimates, but rather are accounted for 

in the estimates of other nations. While these emissions are not directly attributed to the United States, 

they are nevertheless sizeable and important on a global scale. 

10.2 Other Issues/Next Steps 

The results of this analysis are presented to illustrate the potential impact of lifecycle emissions from the 

transportation sector. A number of impacts still need to be addressed to present a more comprehensive 

assessment of the transportation lifecycle. Some of these issues include: 

 Impacts Not Quantified—While this analysis assesses many of the GHG impacts of the 

transportation lifecycle, a significant number of impacts were not quantified. These include 

fuel cycle emissions associated with alternative fuel vehicles (AFVs), and vehicle cycle 

emissions associated with non-road transport. The analysis also did not assess infrastructure 

lifecycle emissions or the land use impacts of transportation, such the removal of trees for 

highway construction, parking lots, airports, and many other types of infrastructure. 

Measuring the latter impacts is extremely challenging. 

 Alternative Fuels and Vehicle Technologies—Resource limitations for this report prevented 

analysis of these fuels and technologies. There is great variance in the lifecycle emissions 

from alternative fuels, and substantial work has been done by others to quantify these 

emissions. Although some of those fuels and vehicle technologies will likely be extremely 

important in the future, their collective use is presently small enough for their contributions to 

have a negligible effect on current lifecycle estimates. Future work should incorporate these 

fuels and technologies because of the critical role they play in forward-looking policy 

analyses. 

 International Boundaries—Accounting for international boundaries could significantly 

increase total transportations sector estimates. In 2001, approximately 55 percent of the 

petroleum products consumed in the United States were derived from crude oil produced 

abroad.103 Supplemental tables may be developed in the future to represent upstream 

emissions occurring outside of the United States. 

103 Energy Information Administration, 2004. International Energy Annual, 2002. Washington, DC. 

40
Transportation GHG Emissions Report 

11 GHG Emissions Projections and Emerging Issues 

11.1 Projected CO2 Emissions from Transportation 

CO2 from transportation is expected to remain a major source of total U.S. greenhouse gas emissions. 

Estimates of transportation energy use are sensitive to factors such as fuel prices, economic growth, and 

technology adoption. In its Annual Energy Outlook (AEO), EIA has developed various scenarios to 

forecast the potential impact of these variables on future fuel consumption.104 In the reference (base) case, 

transportation-related energy demand is projected to increase by 18 percent between 2003 and 2010, and 

by 48 percent by 2025 (Figure 11-1).105 EIA’s high- and low-economic cases show a similar trend (also 

in Figure 11-1). Since CO2 emissions are very highly correlated with fuel consumption, transportationrelated emissions of GHSs are expected to increase at a similar rate. 

Figure 11-1. EIA Projections of Transportation Energy Demand, High, Base, and Low Economic 

Cases, 2003-2025 Quadrillion Btu 

50 

40 

30 

20 

10 

0 

Base

High 

Low 

2003 2005 2010 2015 2020 2025 

Source: Energy Information Administration, U.S. Department of Energy. Annual Energy Outlook 2005 with Projections to 2025. Washington, DC, Table 

B2. 

In the AEO reference case, motor gasoline use is projected to increase by 2.0 percent per year between 

2003 to 2025, from 16.6 to 24.0 quadrillion Btu. Alternative fuels are projected to displace 2.2 percent of 

light-duty vehicle fuel consumption by 2025. Gasoline’s share of demand is nevertheless expected to be 

sustained by low prices relative to the rate of inflation, and a slow increase in the fuel efficiency of 

conventional cars, vans, pickup trucks, and SUVs. Industrial output is assumed to grow 2.3 percent per 

year from 2003 to 2025, leading to continued growth in freight truck use and an annual increase of diesel 

fuel consumption of 2.3 percent. Jet fuel consumption is expected to grow at 1.9 percent annually, 

reflecting growth in passenger travel of 2.2 percent from 2003 to 2025. 

An important assumption underlying the AEO and other forecasts is the continued growth in light-duty 

travel, albeit at a decreasing rate. From 1980 to 2000, light-duty VMT increased at an average annual rate 

of 2.99 percent. EIA forecasts that light-duty VMT will increase by 56 percent between 2003 and 2025, or 

104 See Energy Information Administration, 2005, Annual Energy Outlook (AEO) 2005 with Projections to 2025. 

105 Energy Information Administration, 2005, Annual Energy Outlook (AEO) 2005 with Projections to 2025. Washington, DC, 

Table A2. 

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Transportation GHG Emissions Report 

2.0 percent annually. Meanwhile, EIA projects that the fuel economy of light-duty vehicles will improve 

by about 10 percent over the same period,106 reflecting the planned increases in fuel economy standards 

for light-duty trucks.107 CAFE currently requires that light trucks achieve a manufacturer average of 21.0 

mpg in model year 2005, increasing to 22.2 mpg in 2007. The AEO reference forecast also assumes that 

vehicle technology improvements will marginally improve light-duty fuel economy. 

11.2 Emerging Issues Affecting Passenger Transportation 

Increasing Vehicle Travel 

A number of national-level travel forecasts suggest that the growth in passenger travel will decelerate.108 

There are several reasons to believe that VMT growth could be lower in the future, including an increase 

in the share of elderly drivers and the impact of highway congestion. These forecasts are nonetheless 

speculative, and small variations from the projected annual growth rate of 2.0 percent could be significant 

over time. Annual light-duty VMT growth of 2.5 percent would translate into a 72 percent increase in 

light-duty VMT between 2003 and 2025, or over 1 trillion vehicle miles more in 2025 than the mileage 

implied by a 2.0 percent annual growth rate. 

Consumers’ Vehicle Choice and the Impacts of Light-Duty Trucks 

The growing representation of SUVs and other light-duty trucks in the vehicle fleet is expected to have a 

continuing impact on average in-use vehicle fuel economy. Increased sales of light-duty trucks were 

largely responsible for the decline in new vehicle fuel economy from its peak in the late 1980s. Although 

long-term fuel price changes are uncertain, fuel prices historically have had an effect on vehicle purchase 

decisions and on fuel consumption. Fuel prices have risen significantly since 2003, causing some 

consumers to consider the purchase of vehicles with higher fuel efficiency. Continued price increases of 

this magnitude would likely result in the purchase of more fuel-efficient vehicles. There is significant 

evidence that people respond measurably to changes in fuel prices, with typical reported long-term motor 

fuel price elasticities of -0.5 to -0.8. However, recent studies also suggest that consumers have become 

less sensitive to fuel prices than they were in the past, due in large part to higher average incomes and 

lower real fuel prices as a percentage of household expenses.109 As a result, the effects of fuel prices in 

the near term are uncertain, but may likely be at the lower end of the above elasticity range. The longterm sensitivity to fuel costs is even more uncertain, as are projections of consumers’ future fuel costs. 

Advanced Technology Vehicles 

EIA projections show advanced technology vehicles accounting for 19 percent of light-duty sales in 2025. 

Alcohol flexible-fuel vehicles are expected to comprise about 8 percent of new sales, hybrids about 6 

percent, and turbo direct diesel vehicles about 4 percent. Travel in hybrids also is expected to grow 

significantly from 2003 to 2025 (Figure 11-2), but would still represent less than 5 percent of total lightduty miles in 2025. 

106 Energy Information Administration, 2005. Annual Energy Outlook 2005 with Projections to 2025. Washington, DC, Table 

A7. 

107 Includes all pickup trucks, vans, and SUVs with gross vehicle weight rating less than 8,500 pounds. 

108 As noted above, EIA forecasts light-duty VMT to increase by 2.0 percent from 2003 to 2025; Highway Performance 

Monitoring System (HPMS) forecasts are provided by state departments of transportation and forecast an average increase of 

2.08 percent per year. See U.S. Department of Transportation, Status of the Nation's Highways, Bridges, and Transit: 2002 

Conditions and Performance Report, Chapter 9. Available online at http://www.fhwa.dot.gov/policy/2002cpr/ch9.htm. 

109 Several studies report on the elasticity of fuel consumption with respect to fuel prices. See, for example: Goodwin, Phil. 

“Review of New Demand Elasticities,” Journal of Transport Economics, May 1992. Hagler Bailly, “Potential for Fuel Taxes to 

Reduce Greenhouse Gas Emissions from Transport,” Transportation Table of the Canadian National Climate Change Process, 

1999, and DOE’s Policies and Measures for Reducing Energy-Related Greenhouse Gas Emissions. 

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Transportation GHG Emissions Report 

Increased sales of hybrid vehicles and other advanced technology vehicles offer the potential to reduce 

fuel consumption. Nevertheless, manufacturers could also produce a greater number of less fuel-efficient 

vehicles and remain compliant with current CAFE standards. Another possibility is that hybrid and other 

advanced technology vehicles will be larger and equipped with more powerful engines, causing their fuel 

economy to remain largely unchanged. In either case, the net impact on overall fleet fuel economy could 

be negligible. Hydrogen is a potentially viable alternative to petroleum fuels in the long term. At present, 

the production and storage costs of hydrogen are the major barriers to increased use of hydrogen in 

vehicles. The Department of Energy is involved in several initiatives to increase the use of hydrogen in 

automobiles.110 

Figure 11-2. Historical and Projected VMT from Gasoline- and Diesel-Electric Hybrid Vehicles, 

2000–2025 

160,000 

Vehicle Miles Traveled

(millions) 

140,000 

Diesel-Electric Cars 

120,000 

100,000 

2000 2005 2010 2015 2020 2025 

Light Trucks 

80,000 

Gas-Electric Cars 

60,000 

40,000 

20,000 

0 

Source: Browning, L., 2003. “VMT Projections for Alternative Fueled and Advanced Technology Vehicles through 2025,” 13th CRC On-Road Vehicle 

Emissions Workshop. 

Note: “Light Trucks” category includes both Gas-Electric and Diesel-Electric Vehicles 

State-Level Regulations and Incentives for Vehicle GHG Emissions Reductions 

Although there are currently no national regulations on GHG emissions from motor vehicles, several 

states’ legislatures are beginning to implement or consider controls. Many state and local governments are 

also implementing policies that are placing more attention on GHG emissions and the role of 

transportation. Forty states have already developed GHG inventories, and 28 of them have developed 

detailed climate change action plans. Seven states have set numerical GHG emissions reduction targets, 

and at least four have considered transportation measures in the portfolio of options that will be used to 

achieve those targets. 

California has promulgated regulations to reduce GHG emissions from new vehicles by 22 percent for the 

2012 model year and 30 percent by model year 2016. In 2002, California Assembly Bill 1493 (A.B. 1493) 

was signed into law, charging the California Air Resources Board (CARB) to develop a maximum 

feasible CO2 emissions standard for light-duty vehicles. A second major California GHG initiative was 

launched in May 2005, when California Governor Arnold Schwarzenegger signed an executive order 

committing California to reduce greenhouse gas emissions and achieve increasingly stringent targets. The 

order calls for statewide emissions from all sources in 2010 to meet 2000 levels; emissions in 2020 to 

achieve 1990 levels; and emissions in 2050 to be reduced 80 percent from current levels. 

110 DOE (2005) The Hydrogen Future. Energy Efficiency and Renewable Energy. Available online at 

http://www.eere.energy.gov/hydrogenandfuelcells/future/barriers.html. 

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Transportation GHG Emissions Report 

In May 2005, New York Governor George Pataki proposed regulations similar to the California light-duty 

standard. Beginning with model year 2009, all passenger vehicles registered in New York will be required 

to meet fleet average standards for GHG emissions. This standard would become increasingly stringent 

through 2016. Other states, including Connecticut, Massachusetts, Oregon, and Washington, are 

considering similar regulations. These measures and other state-level actions have the potential to 

influence the energy efficiency of vehicles sold throughout the United States. 

Increasing Demand for Air Travel 

Future GHG emissions from aviation will largely depend on the degree to which improvements in aircraft 

energy efficiency keep pace with growing passenger travel demand. Passenger seat-miles available on 

aircraft are expected to grow 46 percent between 2002 and 2015, and by 67 percent between 2002 and 

2025. Fuel economy of commercial aircraft is expected to increase at a more modest rate, from 54.8 seatmiles per gallon in 2002 to 63.3 in 2015 (an increase of 15.5 percent) and to 67.0 in 2025 (an increase of 

22.3 percent). The result will likely be faster growth in fuel consumption and GHG emissions than 

observed from 1990 to 2003.111 

11.3 Emerging Issues Affecting Freight Transportation 

Freight Trucks—Future Growth in Activity and Changes in Fuel Economy 

GHGs from heavy-duty trucks increased faster than any other major source from 1990 to 2003. Much of 

the growth resulted from rapid increases in freight haulage and vehicle travel, which overwhelmed a 

nominal improvement in vehicle fuel efficiency. A number of trends suggest that similar growth in 

activity is possible in the future. Average shipment sizes have been affected by developments such as the 

growth in e-commerce and direct delivery to end users, which have tended to decrease vehicle loads and 

increase VMT. Meanwhile, it is expected that truck fuel economy will improve marginally. According to 

AEO estimates, the overall fuel efficiency of the freight truck fleet is expected to rise from 6 mpg in 2003 

to 6.6 mpg in 2025.112 The AEO notes that freight companies are sensitive to the marginal costs of 

implementing fuel-efficient strategies and technologies, but anticipates that numerous strategies should 

still penetrate the industry. EIA forecasts that the penetration of these technologies in the freight industry 

will increase new freight truck fuel efficiency from 6.1 mpg in 2003 to 6.8 mpg in 2025.113 

Advanced Technology and Hybrid Vehicles 

In coming years, gasoline- and diesel-electric hybrids will comprise a greater share of urban delivery 

vehicles, although they will likely remain a small proportion in the near term. Hydraulic hybrids represent 

a new technology that may significantly penetrate the heavy-duty vehicle market. Hydraulic hybrid 

vehicles are similar to gasoline-electric hybrids, except that a hydraulic system replaces the battery and 

electric motor. In a hydraulic hybrid, energy from regenerative braking is stored by compressing 

hydraulic fluid in a reservoir. It is used later in a hydraulic pump to provide power to the wheels. Some 

experts believe that larger vehicles, such as pickup trucks and delivery vans, may be able to incorporate 

hydraulic hybrid technology at about the same cost as gasoline-electric systems. 

111 Energy Information Administration, 2004. Annual Energy Outlook 2004, with Projections to 2025. Washington, DC, Table 

7. 

112 Energy Information Administration, 2004. Annual Energy Outlook 2004, with Projections to 2025. Washington, DC, Figure 

57. 

113 Energy Information Administration, 2004. Annual Energy Outlook 2004, with Projections to 2025. Washington, DC, Figure 

58. 

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Transportation GHG Emissions Report 

Programs to Reduce Emissions from Heavy-Duty Vehicles 

Many long-haul trucks idle for extended periods of time, using the engine to power cab amenities. This 

idling is grossly inefficient, and a variety of technologies are available to provide cab heating, cooling, 

and/or electrical supply while consuming less energy. These include direct-fire heaters, auxiliary power 

units, and automatic engine idle systems. Truck stop electrification is another option for reducing truck 

idling that many metropolitan areas are considering as a means to reduce air pollution. These strategies 

would concurrently reduce vehicle fuel consumption and GHG emissions. 

11.4 Implications for the Future 

Forecasts indicate that transportation is likely to remain a major source of total U.S. GHGs, and may be a 

primary contributor to the growth of national greenhouse gas emissions. The AEO 2005 reference case 

scenario shows transportation accounting for the largest absolute increase in energy consumption of any 

U.S. economic sector from 2003 to 2025. Transportation energy consumption is expected to be 

responsible for more than 37 percent of the total increase in U.S. fuel consumption over this period, 

representing an increase of 13.0 quadrillion Btu. While transportation GHGs will be influenced by factors 

such as economic expansion and the cost of energy, a variety of measures may reduce the growth and 

impact of these emissions. Broadly categorized, these measures could include efforts to encourage 

energy-efficient vehicle technologies, promote efficient patterns of travel and land use, and develop 

alternatives to petroleum-based fuels. The timing and implementation of such approaches will 

significantly affect the future volume of greenhouse gases from U.S. transportation sources. 

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Transportation GHG Emissions Report 

12 References 

American Association of State Highway and Transportation Officials. Freight-Rail Bottom Line Report, 

Washington, DC. 

American Trucking Associations (2004) U.S. Freight Transportation Forecast to 2014, Alexandria, VA. 

Association of American Railroads (2004) Railroad Facts. Washington, DC. 

Browning, L. (2003). “VMT Projections for Alternative Fueled and Advanced Technology Vehicles 

through 2025,” 13th CRC On-Road Vehicle Emissions Workshop, April 2003. 

Delucchi, M (2003). Lifecycle Emissions Model (LEM), Mark A. Delucchi, Institute of Transportation 

Studies, University of California, December 2003. 

Delucchi, M (2003) Personal Communication between Mark Delucchi of the University of California and 

Bill Cowart of ICF Consulting. June 14, 2003. 

Center for Transportation Research (2001) Greenhouse Gases, Regulated Emissions, and Energy Use in 

Transportation (GREET) 1.6 Model. Argonne National Laboratory, U.S. Department of Energy, 

Argonne, IL, June 2001. 

DOE (2004) Transportation Energy Data Book, Edition 24. Center for Transportation Analysis, 

Engineering Science and Technology Division, Oak Ridge National Laboratory, ORNL-6973. 

Available online at http://www-cta.ornl.gov/data/Index.html. 

DOE (2005) The Hydrogen Future. Energy Efficiency and Renewable Energy. Available online at 

http://www.eere.energy.gov/hydrogenandfuelcells/future/barriers.html. 

DOT (2005) Transportation Statistics Annual Report 2005. Bureau of Transportation Statistics, U.S. 

Department of Transportation, Washington, DC. Available online 

athttp://www.bts.gov/publications/transportation_statistics_annual_report/. 

Transportation Statistics, U.S. Department of Transportation, Washington, DC. Available online at 

http://www.bts.gov/publications/national_transportation_statistics/2004/index.html. 

DOT (2004) National Transportation Statistics 2003. Bureau of Transportation Statistics, U.S. 

Department of Transportation, Washington, DC. Available online at 

http://www.bts.gov/publications/national_transportation_statistics/2003/index.html. 

EIA (2005) Annual Energy Outlook 2005 with Projections to 2025. Energy Information Administration, 

U.S. Department of Energy, Washington, DC. Available online at 

http://www.eia.doe.gov/oiaf/archive/aeo05/index.html. 

EIA (2004) Annual Energy Outlook 2004 with Projections to 2025. Energy Information Administration, 

U.S. Department of Energy, Washington, DC. Available online at 

http://www.eia.doe.gov/oiaf/archive/aeo04/index.html. 

46
Transportation GHG Emissions Report 

EIA (2005) Household Vehicles Energy Use: Latest Data & Trends, Energy Information Administration, 

U.S. Department of Energy, Washington, DC. Available online at 

http://www.eia.doe.gov/emeu/rtecs/nhts_survey/2001/index.html. 

EIA (2004) International Energy Annual, 2002 Edition. Energy Information Administration, U.S. 

Department of Energy, Washington, DC. Available online at 

http://www.eia.doe.gov/emeu/iea/contents.html. 

EPA (2005) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003. U.S. Environmental 

Protection Agency, Washington, DC. Available online at 

http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsGHGEmissio 

nsUSEmissionsInventory2005.html. 

EPA (2005) Light-Duty Automotive Technology and Fuel Economy Trends, 1975 through 2005. Office of 

Transportation and Air Quality, U.S. Environmental Protection Agency, Washington, DC. 

Available online at http://www.epa.gov/otaq/fetrends.htm. 

EPA (2004) U.S. EPA Vintaging Model. Version VM IO file_11-08-04. 

FHWA (2002 through 2004) Highway Statistics. Federal Highway Administration, U.S. Department of 

Transportation, Washington, DC, report FHWA-PL-96-023-annual. Available online at 

http://www.fhwa.dot.gov/policy/ohim/hs03/index.htm and 

http://www.fhwa.dot.gov/ohim/hs01/index.htm. 

FHWA (1997) Highway Statistics: Summary to 1995. Federal Highway Administration, U.S. Department 

of Transportation, Washington, DC. Available online at 

http://www.fhwa.dot.gov/ohim/summary95/index.html. 

FHWA (2004) Summary of Travel Trends: 2001 Nationwide Household Transportation Survey. Federal 

Highway Administration, U.S. Department of Transportation, Washington, DC. Available online 

at http://nhts.ornl.gov/2001/pub/STT.pdf. 

FHWA (1999) Summary of Travel Trends: 1995 Nationwide Personal Transportation Survey. Federal 

Highway Administration, U.S. Department of Transportation, Washington, DC, report FHWAPL00-006. Available online at http://npts.ornl.gov/npts/1995/Doc/trends_report.pdf. 

First Fleet Corporation (2004) Fleet Managers’ Survey, Ft. Lauderdale, FL. Spring. Available online at 

http://ffcsurvey.starmark.com/FFC_Executive_Report.pdf?-

session=thisSession:96EC3B6CBA05324EEB617B369C2AF347. 

Goodwin, Phil. Review of New Demand Elasticities, Journal of Transport Economics and Policy, 

Volume 26, May 1992. 

Hagler Bailly, Potential for Fuel Taxes to Reduce Greenhouse Gas Emissions from Transport, 

Transportation Table of the Canadian National Climate Change Process, 1999. 

IPCC (2001) Climate Change 2001: The Scientific Basis, Intergovernmental Panel on Climate Change; 

J.T. Houghton, L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg, and K. Maskell, eds.; 

Cambridge University Press. Cambridge, U.K. 

47
Transportation GHG Emissions Report 

U.S. Census Bureau (2003) Decennial Census, Supplemental Survey: Journey to Work. Department of 

Commerce, Washington, DC. Reported in A. Pisarski, Commuting in America III. Eno River 

Foundation. 

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Transportation GHG Emissions Report 

13Appendix A: Summary of GHG Emissions for Transportation and Mobile Sources This appendix contains summary tables with estimates of CO2, CH4, N2O, and HFCs from transportation and non-transportation sou

Table 13-1. Total GHG Emissions from Transportation Sources (All Gases), 1990-2003 (Tg CO2 Eq.) 

Fuel/Vehicle 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 

On-Road 1,196.1 1,190.8 1,214.0 1,246.2 1,276.5 1,304.7 1,338.9 1,363.2 1,402.9 1,442.3 1,460.3 1,468.7 1,490Passenger Cars 640.6 605.1 604.0 613.2 618.7 621.6 627.3 624.1 642.3 650.0 649.7 650.2 662Light-Duty Trucks 

327.7 359.5 379.8 395.0 404.4 419.9 433.6 446.6 457.1 473.9 476.2 477.7 487

Medium/HeavyDuty Trucks 217.9 216.8 220.8 227.9 242.9 252.9 267.1 281.4 292.1 305.9 322.1 329.2 329Buses 8.2 7.7 7.8 8.3 8.7 8.6 9.1 9.4 9.6 10.6 10.5 9.9 9Motorcycles 1.7 1.7 1.7 1.8 1.8 1.8 1.7 1.8 1.8 1.8 1.8 1.7 1Aircraft 179.1 171.2 168.9 169.9 178.0 173.6 182.0 180.9 183.2 188.7 195.2 185.3 176General Aviation Aircraft 

9.5 8.5 7.6 6.8 7.1 8.1 8.4 8.9 10.3 12.0 11.8 11.6 11

Commercial Aircraft 

118.4 110.5 112.9 114.7 118.6 121.3 126.5 129.8 127.6 137.9 142.1 134.2 123

Military Aircraft 35.1 34.9 28.5 27.9 25.3 24.4 23.3 21.2 21.7 20.8 21.2 23.1 20Other Aircraft 16.1 17.3 19.8 20.5 26.9 19.8 23.8 20.9 23.6 18.0 20.1 16.6 21Boats and Ships 

49.6 44.2 57.7 54.7 53.9 55.7 53.9 39.1 32.7 42.7 63.7 43.2 57

Locomotives 36.6 35.0 35.1 35.2 38.0 39.4 41.1 40.4 40.9 42.1 42.2 43.3 41Pipelines 35.9 32.7 32.1 33.9 37.3 38.0 38.7 40.9 34.9 35.3 35.0 33.4 36Lubricants 11.9 10.6 10.8 11.0 11.5 11.3 11.0 11.6 12.1 12.3 12.1 11.1 10Mobile AC + + 0.6 2.0 4.3 6.6 10.1 13.8 17.4 20.8 24.0 26.7 28Refrigerated Transport 

+ + + 0.3 0.9 2.3 3.8 5.5 7.0 8.5 9.8 10.8 11

Total 1,509.3 1,484.5 1,519.3 1,553.2 1,600.4 1,631.8 1,679.5 1,695.4 1,731.1 1,792.5 1,842.3 1,822.4 1,853+ Less than 0.05 Tg CO2 Eq. 

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Transportation GHG Emissions Report 

Table 13-2. CO2 Emissions from Transportation Sources, 1990-2003 (Tg) 

Mode / Vehicle 

Type / Fuel 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 

Highway Vehicles 

1,151.3 1,143.5 1,164.0 1,194.3 1,223.1 1,250.4 1,284.0 1,307.8 1,347.4 1,388.0 1,408.0 1,420.7 1,445.8 

Passenger Cars 

612.5 577.6 575.8 584.8 589.8 592.5 598.4 595.5 613.8 622.4 623.4 625.7 639.5 

Light-Duty Trucks 

312.2 341.3 359.4 373.0 381.5 396.2 409.3 421.6 432.1 449.2 452.1 456.2 468.

Medium/HeavyDuty Trucks 217.0 215.6 219.6 226.7 241.7 251.6 265.7 279.9 290.4 304.3 320.4 327.5 327.5 Buses 7.8 7.3 7.5 8.0 8.4 8.3 8.8 9.1 9.3 10.4 10.2 9.6 9.Motorcycles 1.7 1.6 1.7 1.7 1.8 1.7 1.7 1.7 1.8 1.8 1.8 1.6 1.6 Aircraft 177.2 169.4 167.1 168.1 176.1 171.8 180.1 179.0 181.3 186.7 193.2 183.4 174.9 General Aviation Aircraft 

9.4 8.3 7.5 6.7 7.0 8.0 8.3 8.8 10.1 11.8 11.7 11.4 11.6 

Commercial Aircraft 

117.2 109.4 111.8 113.5 117.4 120.1 125.2 128.5 126.3 136.4 140.6 132.8 121.7 

Military Aircraft 34.8 34.5 28.2 27.6 25.0 24.1 23.1 21.0 21.5 20.6 21.0 22.8 20.4 Other Aircraft 15.9 17.1 19.6 20.3 26.7 19.6 23.5 20.6 23.4 17.8 19.9 16.4 21.2 Boats and Ships 

49.2 43.7 57.2 54.2 53.4 55.2 53.4 38.7 32.4 42.3 63.1 42.7 57.2 

Locomotives 36.3 34.7 34.8 34.8 37.6 39.1 40.7 40.0 40.5 41.7 41.8 42.8 41.0 Pipelines 35.9 32.7 32.1 33.9 37.3 38.0 38.7 40.9 34.9 35.3 35.0 33.4 36.4 Lubricants 11.9 10.6 10.8 11.0 11.5 11.3 11.0 11.6 12.1 12.3 12.1 11.1 10.9 TOTAL 1,461.7 1,434.7 1,466.0 1,496.4 1,539.1 1,565.8 1,607.9 1,618.0 1,648.7 1,706.2 1,753.1 1,734.2 1,766.4 

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Transportation GHG Emissions Report 

Table 13-3. Methane Emissions from Transportation Sources, 1990-2003 (Tg CO2 Eq.) 

Mode / Vehicle 

Type / Fuel 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 

Highway Vehicles 

4.3 4.3 4.2 4.1 4.0 3.9 3.7 3.6 3.4 3.1 2.8 2.6 2.

Passenger Cars 

2.6 2.4 2.3 2.3 2.2 2.1 2.0 1.9 1.8 1.7 1.5 1.4 1.

Light-Duty Trucks 

1.4 1.5 1.5 1.5 1.5 1.4 1.4 1.3 1.3 1.1 1.0 0.9 0.

Medium/HeavyDuty Trucks + + + + + + + + + + + + Buses 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.Motorcycles + + + + + + + + + + + + Aircraft 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.2 0.2 0.1 0.General Aviation Aircraft 

0.1 0.1 0.1 + + 0.1 + 0.1 + 0.1 0.1 + 

Commercial Aircraft 

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.

Military Aircraft + + + + + + + + + + + + Other Aircraft + + + + + + + + + + + + Boats and Ships 

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.

Locomotives 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.Total 4.6 4.5 4.5 4.4 4.3 4.2 4.0 3.8 3.7 3.4 3.2 2.9 2.+ Less than 0.05 Tg CO2 Eq. 

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Transportation GHG Emissions Report 

 Table 13-4. Nitrous Oxide Emissions from Transportation Sources, 1990-2003 (Tg CO2 Eq.) 

Mode / Vehicle 

Type / Fuel Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 

Highway Vehicles 40.6 43.0 45.8 47.8 49.3 50.4 51.2 51.9 52.1 51.2 49.5 45.4 42.0 Passenger Cars 25.5 25.1 25.8 26.1 26.7 26.9 26.9 26.7 26.7 25.9 24.7 23.1 21.6 Light-Duty Trucks 14.1 16.7 18.8 20.4 21.4 22.2 22.9 23.7 23.7 23.6 23.0 20.6 18.6 Medium/HeavyDuty Trucks 0.9 1.2 1.1 1.1 1.2 1.3 1.4 1.4 1.6 1.6 1.7 1.7 1.8 Buses + + + + + + + + + + + + + Motorcycles + + + + + + + + + + + + + Aircraft 1.7 1.6 1.6 1.6 1.7 1.7 1.8 1.7 1.8 1.8 1.9 1.8 1.7 General Aviation Aircraft 

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Commercial Aircraft 

1.1 1.1 1.1 1.1 1.2 1.2 1.2 1.3 1.2 1.3 1.4 1.3 1.2 

Military Aircraft 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Other Aircraft 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Boats and Ships 0.4 0.3 0.5 0.4 0.4 0.4 0.4 0.3 0.3 0.3 0.5 0.3 0.5 Locomotives 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Total 43.0 45.3 48.1 50.1 51.8 52.9 53.7 54.3 54.4 53.7 52.2 47.9 44.5 + Less than 0.05 Tg CO2 Eq. Table 13-5. HFC Emissions from Transportation Sources, 1990-2002 (Tg CO2 Eq.) 

Tg CO2 Eq. 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 200

Mobile AC + + 0.6 2.0 4.3 6.6 10.1 13.8 17.4 20.8 24.0 26.7 28.8 3Refrigerated Transport + + - 0.3 0.9 2.3 3.8 5.5 7.0 8.5 9.8 10.8 11.5 1Total + + 0.6 2.3 5.2 8.9 13.9 19.4 24.4 29.3 33.8 37.4 40.4 4

+ Less than 0.05 Tg CO2 Eq. 

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Transportation GHG Emissions Report 

Table 13-6. GHG Emissions from Non-Transportation Mobile Sources (All Gases), 1990-2003 (Tg CO2 Eq.) 

Fuel Type/Vehicle 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 

Farm Eq. 30.5 31.1 32.3 42.3 35.0 36.0 36.8 38.3 38.5 37.6 38.0 40.2 41.3 Construction Eq. 39.0 40.2 41.4 56.3 44.2 45.6 47.0 48.5 49.3 50.1 51.6 55.8 57.4 Industrial and Commercial Eq. 9.8 9.9 10.0 9.8 9.0 9.4 9.5 9.8 10.3 9.3 9.6 16.0 16.6 Lawn and Garden Eq. 12.1 12.5 12.9 14.5 13.6 13.9 14.6 13.7 13.7 13.9 14.1 14.2 14.2 Recreational Eq. 6.6 6.6 6.6 9.0 6.6 6.6 6.5 6.7 6.9 7.2 7.5 7.9 8.4 Other* 2.6 2.6 2.6 2.8 2.7 2.7 2.7 2.7 2.7 2.6 2.7 2.7 2.8 Total 100.7 102.9 105.8 134.8 111.0 114.0 117.1 119.6 121.3 120.7 123.6 136.8 140.6 * “Other" includes logging equipment, railroad equipment, and airport equipment. Table 13-7. CO2 Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg) 

Fuel 

Type/Vehicle 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2

Farm Equipment 30.18 30.75 31.91 41.88 34.61 35.58 36.39 37.86 38.06 37.18 37.64 39.76 40.84 4Construction Equipment 38.69 39.79 41.03 55.80 43.76 45.17 46.55 48.04 48.82 49.67 51.14 55.27 56.88 5Industrial and Commercial Eq. 9.71 9.84 9.93 9.76 8.93 9.28 9.46 9.71 10.21 9.22 9.49 15.83 16.42 1Lawn and Garden Eq. 12.03 12.40 12.77 14.39 13.47 13.75 14.48 13.62 13.56 13.76 14.02 14.07 14.11 1Recreational Eq. 6.56 6.56 6.57 8.94 6.56 6.52 6.48 6.61 6.81 7.10 7.47 7.84 8.31 Other* 2.59 2.60 2.61 2.77 2.63 2.64 2.65 2.66 2.67 2.61 2.69 2.72 2.74 Total 99.76 101.94 104.82 133.54 109.96 112.94 116.01 118.49 120.15 119.54 122.43 135.48 139.31 14* “Other" includes logging equipment, railroad equipment, and airport equipment. 

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Transportation GHG Emissions Report 

Table 13-8. Methane Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg CO2 Eq.) 

Fuel 

Type/Vehicle 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 20

Farm Equipment 0.091 0.092 0.096 0.126 0.104 0.107 0.110 0.114 0.115 0.112 0.113 0.120 0.123 0Construction Equipment 

0.046 0.048 0.049 0.067 0.053 0.054 0.056 0.058 0.059 0.060 0.061 0.066 0.068 0

Industrial and Commercial Eq. 

0.012 0.012 0.012 0.012 0.011 0.011 0.011 0.012 0.012 0.011 0.011 0.019 0.020 0

Lawn and Garden Eq. 

0.014 0.015 0.015 0.017 0.016 0.017 0.017 0.016 0.016 0.017 0.017 0.017 0.017 0

Recreational Eq. 0.008 0.008 0.008 0.011 0.008 0.008 0.008 0.008 0.008 0.009 0.009 0.009 0.010 0Other* 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0Total 0.174 0.178 0.183 0.236 0.194 0.200 0.205 0.211 0.213 0.211 0.215 0.235 0.242 0* “Other" includes logging equipment, railroad equipment, and airport equipment. Table 13-9. Nitrous Oxide Emissions from Non-Transportation Mobile Sources, 1990-2003 (Tg CO2 Eq.) 

Fuel 

Type/Vehicle 

Type 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 

Farm Equipment 0.238 0.242 0.251 0.330 0.273 0.281 0.287 0.299 0.301 0.294 0.297 0.314 0.323 Construction Equipment 

0.305 0.314 0.323 0.440 0.345 0.356 0.367 0.379 0.385 0.392 0.403 0.436 0.449 

Industrial and Commercial Eq. 

0.077 0.078 0.078 0.077 0.070 0.073 0.075 0.077 0.081 0.073 0.075 0.126 0.131 

Lawn and Garden Eq. 

0.094 0.097 0.100 0.112 0.105 0.108 0.114 0.107 0.107 0.109 0.111 0.111 0.111 

Recreational Eq. 0.051 0.051 0.051 0.070 0.051 0.051 0.051 0.052 0.054 0.056 0.059 0.062 0.066 Other* 0.021 0.021 0.021 0.022 0.021 0.021 0.021 0.021 0.021 0.021 0.021 0.022 0.022 Total 0.785 0.802 0.825 1.051 0.866 0.892 0.916 0.936 0.949 0.944 0.967 1.071 1.101 * “Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment, commercial equipment, and indu

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Transportation GHG Emissions Report 

14 Appendix B: CO2 Emissions from Various Components of the Transportation 

(Proportion Relative to Direct Emissions) 

Table 14-1. CO2 Emissions from Various Components of the Transportation Lifecycle (Proportion Relative to Direc

Direct Fuel Cycle Vehicle Manufacture Cycle 

Direct Extraction Shipment Refining Distri- bution Sub-Total Non- transport Transport Sub- Total 

Highway Vehicles 

Passenger Cars 

Conv Gasb 

1.00 0.05 - 0.09 0.01 - 0.02 0.18 -

0.19 

0.01 0.24 - 0.31 0.12 -

0.15 

0.02 - 0.03 0.14 -

0.19 

US RFG 1.00 0.05 - 0.08 0.01 - 0.02 0.19 0.01 0.25 - 0.30 0.12 -

0.15 

0.02 - 0.03 0.14 -

0.18 

Diesel 1.00 0.04 - 0.08 0.01 - 0.02 0.09 -

0.13 

0.01 0.15 - 0.25 0.11 -

0.17 

0.03 - 0.04 0.14 -

0.21 

AFVs 1.00 

Light-Duty Trucks 

Conv Gas 

1.00 0.05 - 0.09 0.01 - 0.02 0.18 -

0.19 

0.01 0.24 - 0.31 0.12 -

0.15 

0.02 - 0.03 0.14 -

0.19 

US RFG 1.00 0.05 - 0.08 0.01 - 0.02 0.19 0.01 0.25 - 0.30 0.12 -

0.15 

0.02 - 0.03 0.14 -

0.18 

Diesel 1.00 0.04 - 0.08 0.01 - 0.02 0.0 9 -

0.13 

0.01 0.15 - 0.25 0.11 -

0.17 

0.03 - 0.04 0.14 -

0.21 

AFVs 1.00 

Medium/ Heavy-Duty Trucks 

Conv Gas 

1.00 0.05 - 0.09 0.01 - 0.02 0.18 -

0.19 

0.01 0.24 - 0.31 0.05 -

0.15 

0.01 - 0.02 0.06 -

0.17 

US RFG 1.00 0.05 - 0.08 0.01 - 0.02 0.19 0.01 0.25 - 0.3 0.05 -

0.15 

0.01 - 0.02 0.06 -

0.17 

Diesel 1.00 0.04 - 0.08 0.01 - 0.02 0.09 -

0.13 

0.01 0.15 - 0.25 0.04 -

0.17 

0.02 - 0.03 0.06 -

0.20 

AFVs 1.00 

Buses 

Conv Gas 

1.00 0.05 - 0.09 0.01 - 0.02 0.18 -

0.19 

0.01 0.24 - 0.31 0.12 -

0.15 

0.02 - 0.03 0.14 -

0.19 

US RFG 1.00 0.05 - 0.08 0.01 - 0.02 0.19 0.01 0.25 - 0.3 0.12 -

0.15 

0.02 - 0.03 0.14 -

0.18 

Diesel 1.00 0.04 - 0.08 0.01 - 0.02 0.09 -

0.13 

0.01 0.15 - 0.25 0.04 -

0.17 

0.02 - 0.03 0.06 -

0.20 

AFVs 1.00 

Motorcycles 

Conv Gas 

1.00 0.05 - 0.09 0.01 - 0.02 0.18 -

0.19 

0.01 0.24 - 0.31 0.00 -

0.15 

0.00 - 0.02 0.00 -

0.17 

US RFG 1.00 0.05 - 0.08 0.01 - 0.02 0.19 0.01 0.25 - 0.30 0.00 -

0.15 

0.00 - 0.02 0.00 -

0.17 

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Transportation GHG Emissions Report 

Direct Fuel Cycle Vehicle Manufacture Cycle 

Direct Extraction Shipment Refining Distri- bution Sub-Total Non- transport Transport Sub- Total 

Aircraft General 

Aviation Aircraft 

Jet Fuel 1.00 0.05 - 0.10 0.01 - 0.02 0.11 0.01 0.17 - 0.24 

Aviation Gasoline 

1.00 0.05 - 0.08 0.01 - 0.02 0.19 0.01 0.25 - 0.30 

Commercial Aircraft 

Jet Fuel 1.00 0.05 - 0.10 0.01 - 0.02 0.11 0.01 0.17 - 0.24 

Military Aircraft 

Jet Fuel 1.00 0.05 - 0.10 0.01 - 0.02 0.11 0.01 0.17 - 0.24 

Other Aircraft Jet Fuel 1.00 0.05 - 0.10 0.01 - 0.02 0.11 0.01 0.17 - 0.24 

International (Bunkers) 

Jet Fuel 1.00 0.05 - 0.10 0.01 - 0.02 0.11 0.01 0.17 - 0.24 

Boats and Ships 

Domestic 

Conv Gas 

1.00 0.05 - 0.09 0.01 - 0.02 0.18 -

0.19 

0.01 0.24 - 0.31 

Distillate Fuel 

1.00 0.05 - 0.09 0.01 - 0.02 0.10 -

0.11 

0.01 0.16 - 0.23 

Residual Fuel 

1.00 0.04 - 0.08 0.01 - 0.02 0.05 0.01 0.10 - 0.16 

International (Bunkers) 

Distillate Fuel 

1.00 0.05 - 0.09 0.01 - 0.02 0.10 -

0.11 

0.01 0.16 - 0.23 

Residual Fuel 

1.00 0.04 - 0.08 0.01 - 0.02 0.05 0.01 0.10 - 0.16 

Rail Distillate 

Fuel 

1.00 0.05 - 0.09 0.01 - 0.02 0.10 -

0.11 

0.01 0.16 - 0.23 

Electricity 1.00 0.03 - 0.04 0.00 1.00 -

1.09 

0.00 1.03 - 1.12 

Pipelines Natural 

Gas 

1.00 0.03 - 0.04 0.00 0.05 -

0.11 

0.01 -0.05 

0.09 - 0.20 

Electricity 1.00 0.03 - 0.04 0.00 1.00 -

1.09 

0.00 1.03 - 1.12 

Total 

a A “total” value is not calculated because it would be double-counting some transport emissions. b Conv gas = convention gasoline; US RFG = reformulated gasoline; AFVs = alternative fuel vehicles 

Note: The range in each cell is determined by the values provided by GREET and LEM. In some cases, GREET provided the lower values, while in other cases, LEM provided 

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Transportation GHG Emissions Report 

Table 14-2. Total CO2 Emissions from Various Components of the Transportation Lifecycle (Tg) 

Mode Vehicle Fuel Direct Emissions Indirect Emissions Total Lifecycle Emissions Total Emissions, Excluding Transport 

Conv Gasa 395.2 151 - 197 546.2 - 592.1 532.3 - 565.4 

Passenger US RFG 235.1 91.9 - 111.3 327.0 - 346.4 318.6 - 331.5 Cars Diesel 3.4 1 - 1.6 4.4 – 5.0 4.3 - 4.8 

AFVs + – + –+ –+ Conv. Gas 289.0 110.5 – 144.0 399.4 – 433.0 389.3 - 413.5 

Light-Duty US RFG 171.9 67.2 - 81.4 239.1 - 253.3 233 - 242.4 Trucks Diesel 17.6 5.2 - 8.1 22.8 - 25.7 22 - 24.4 

AFVs 0.3 –+ 0.3 0.3 

On-Road Medium/ Conv. Gas 24.8 7.4 - 12 32.2 - 36.8 31.5 - 35.5 US RFG 14.8 4.5 - 6.8 19.3 - 21.6 18.9 - 20.8 

Trucks Heavy Duty Diesel 301.1 63.2 - 134.4 364.3 - 435.5 355 - 417.6 

AFVs 0.5 +– 0.5 0.5 Conv. Gas 0.2 0.1 0.3 0.3 

Buses US RFG 0.1 0.1 0.2 0.2 

Diesel 8.0 1.7 - 3.6 9.7 - 11.6 9.4 - 11.1 AFVs 0.6 –+ 0.6 0.6 Conv. Gas 1.0 0.2 - 0.5 1.3 - 1.5 1.2 - 1.5 Motorcycles US RFG 0.6 0.1 - 0.3 0.8 - 0.9 0.7 - 0.9 

General Jet Fuel 9.4 1.6 - 2.3 10.9 - 11.6 10.8 - 11.3 Aviation Aircraft 

Aviation Gasoline 2.2 0.6 - 0.7 2.8 - 2.9 2.8 - 2.8 

Aviation 

Commercial Aircraft Jet Fuel 122.8 20.4 - 29.6 143.2 - 152.3 141.3 - 148.3 Military Aircraft Jet Fuel 20.5 3.4 - 4.9 24 - 25.5 23.6 - 24.8 Other Aircraft Jet Fuel 16.3 2.7 - 3.9 19.1 - 20.3 18.8 - 19.7 International (Bunkers) Jet Fuel 59.6 9.9 - 14.3 69.5 - 73.9 68.6 - 71.9 

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Transportation GHG Emissions Report 

Mode Vehicle Fuel Direct Emissions Indirect Emissions Total Lifecycle Emissions Total Emissions, Excluding Transport 

Conv. Gas 11.0 2.6 - 3.5 13.7 - 14.5 13.5 - 14.1 

Domestic 

Distillate Fuel 17.0 2.7 - 4 19.7 - 20.9 19.4 - 20.4 

Waterborne Residual 

Fuel 29.5 3.1 - 4.8 32.5 - 34.3 32.1 - 33.4 Distillate 

International Fuel 6.0 1.0 - 1.4 7 - 7.4 6.9 - 7.2 (Bunkers) Residual 

Fuel 18.6 1.9 - 3.1 20.6 - 21.7 20.3 - 21.1 

Rail Locomotives and Transit 

Distillate Fuel 39.6 6.4 - 9.2 45.9 - 48.8 45.3 - 47.6 Electricity 3.2 + 3.2 3.2 

Pipelines 

Natural Gas 34.8 3.1 - 6.8 37.9 - 41.6 37.6 - 39.9 Electricity + + + –+ 

Lubricants All oils 10.2 0 - 0 10.2 - 10.2 10.2 - 10.2 

TOTAL 1,864.9 563.6 - 789.7 b 2,372.7 – 2,547.3 

Total % 1.27% - 1.37% a Not estimated to avoid double-counting transport-related emissions b Conv gas = convention gasoline; US RFG = reformulated gasoline; AFVs = alternative fuel vehicles 

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Transportation GHG Emissions Report 

15Abbreviations, Acronyms, and Units 

AAR Association of American Railroads 

AEO Annual Energy Outlook 

AFV Alternative fuel vehicle 

APTA American Public Transportation Association 

Btu British thermal unit 

0

C Degree Celsius 

CAFE Corporate Average Fuel Economy 

CARB California Air Resources Board 

CFC Chlorofluorocarbon 

CH4 Methane 

CNG Compressed Natural Gas 

CO Carbon monoxide 

CO2 Carbon dioxide 

CO2 Eq. Carbon dioxide equivalent 

DOE U.S. Department of Energy 

DOT U.S. Department of Transportation 

EIA Energy Information Agency 

EPA U.S. Environmental Protection Agency 

0

F Degree Fahrenheit 

FHWA Federal Highway Administration 

GHG Greenhouse gas 

GREET Greenhouse Gases, Regulated Emissions, and Energy Use in 

Transportation 

GWP Global warming potential 

HCFC Hydrochlorofluorocarbon 

HFC Hydrofluorocarbon 

HPMS Highway Performance Monitoring System 

H2O Water 

IPCC Intergovernmental Panel on Climate Change 

lbs Pounds 

LC Lifecycle 

LCA Lifecycle assessment 

LEM Lifecycle Emissions Model 

LEV Low Emissions Vehicle 

LPG Liquefied petroleum gas 

mpg Miles per gallon 

MMBtu Million British thermal units 

mph Miles per hour 

MTA Metropolitan Transportation Authority 

NEI National Emission Inventory 

NHTS National Household Travel Survey 

NHTSA National Highway Traffic Safety Administration 

N2O Nitrous oxide 

59
Transportation GHG Emissions Report 

NOx Oxides of nitrogen 

NPTS Nationwide Personal Travel Survey 

ORNL Oak Ridge National Laboratory 

OTAQ Office of Transportation and Air Quality 

PFC Perfluorocarbon 

PPM Parts per million 

RFG Reformulated gasoline 

RV Recreational vehicle 

SF6 Sulfur hexafluoride 

SO2 Sulfur dioxide 

SUV Sport utility vehicle 

Tg CO2 Eq. Teragrams carbon dioxide equivalent 

TIUS Truck Inventory and Use Survey 

VIUS Vehicle Inventory and Use Survey 

VMT Vehicle miles traveled 

VOC Volatile organic compound 

Conversions 

1 Tg = 1 MMT (million metric ton) 

1 Tg = 1x1012 grams 

1 metric ton = 1,000 kilograms = 1.1023 short tons 

1 pound = 0.454 kilograms 

1 gallon = 3.785412 liters 

1 mile = 1.609 kilometers 

To convert degrees Fahrenheit to degrees Celsius, subtract 32 and multiply by 5/9. 

60