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Freight Movement & Air Quality


Executive Summary

Chapter 1 - Introduction

Chapter 2 - National Freight Transportation Trends and Emissions

2.1 Freight Transportation Activity and Trends
2.2 Freight Transportation Forecasts
2.3 Effects of Emission Standards
2.4 National Freight Transportation Emissions

Chapter 3 - Freight Transportation Emissions at the Regional Level

3.1 Regional Freight Activity
3.2 Trucking Emissions
3.3 Freight Railroad Emissions
3.4 Marine Freight Emissions
3.5 Air Freight Emissions
3.6 Summary and Comparison

Chapter 4 - Emission Mitigation Strategies

4.1 Technological Strategies
4.2 Operational Strategies

Chapter 5 - Conclusions and Recommendations

Appendix A - EPA Emission Standards

Appendix B - Estimation of Future Truck Emissions

Appendix C - Regional Freight Transportation Profiles


Executive Summary

The U.S. economy is dependent on an efficient and reliable freight transportation system. Our highways, ports, waterways, railways, airports, and intermodal facilities make up a complex system that shippers rely on to move products to markets. The performance of that system has direct implications for the productivity of the U.S. and regional economies, the costs of goods and services, and the global competitiveness of our industries. Yet, there is significant and growing concern on the part of both the private and public sectors about the future performance of our freight transportation system. Demand for freight transportation has been rising steadily and forecasts show continued growth over at least the next several decades, while expansion of freight system capacity has been relatively limited.

Prompted by these trends, federal, state, and local agencies are undertaking a variety of initiatives to ensure that the performance of the nation's freight system does not significantly deteriorate. These initiatives include new efforts to fund freight system improvements and efforts to mainstream freight into the transportation planning and programming process. As freight becomes more integrated with overall transportation decision making, there is greater need to consider the air quality impacts of freight at all stages of planning and project development.

Over the last two decades, freight has become a more significant source of air pollution. One reason for this is the robust growth in freight activity, particularly trucking, intermodal rail, foreign waterborne shipments, and air cargo. The other factor is the relatively less stringent regulation on emissions from the freight sector, particularly emissions from locomotives and marine vessels.

At the same time that freight transportation is growing in its contribution to air pollution, there is a heightened concern about the health and environmental effects of diesel engine emissions. Most freight trucks, locomotives, and ships are powered by diesel engines, which are a major source of emissions of nitrogen oxides (NOx) and particulate matter (PM). Freight transportation is also a large and growing source of greenhouse gas (GHG) emissions that contribute to global climate change. These concerns, and the implementation of the 8-hour ozone and fine particulate (PM-2.5) standards, will require many regions across the country to find new ways to control NOx and PM emissions from freight transportation sources.

This study is intended to help fill a void in the current understanding of the air quality impacts of freight transportation. This report discusses freight transportation activity and emissions at the national level and in six metropolitan areas (Baltimore, Chicago, Dallas-Fort Worth, Detroit, Houston, and Los Angeles). The report draws on a variety of existing studies and data sources and develops new emissions estimates to fill data gaps.

Summary of National Freight Transportation Emissions

This study shows that freight is a major source of national NOx and PM-10 emissions. As illustrated in Table ES-1, freight transportation accounts for approximately half of mobile source NOx emissions and 27 percent of all NOx emissions at the national level. Freight transportation accounts for 36 percent of U.S. mobile source PM-10 emissions and less than 1 percent of all U.S. PM-10 emissions. (The vast majority of PM-10 emissions comes from agricultural fields, wildfires, and fugitive dust.)

Heavy-duty vehicles (trucks) are by far the largest contributor to freight emissions nationally, producing two-thirds of the NOx and PM-10 from the freight sector. Marine vessels are the next largest source, accounting for 18 percent of freight NOx emissions and 24 percent of freight PM-10 emissions, followed by railroads at 15 percent of NOx and 12 percent of PM-10. Air freight accounts for only 0.1 to 0.2 percent of total freight emissions of NOx and PM-10, respectively.

Table ES-1: U.S. Freight Transportation NOx and PM-10 Emissions by Mode, 2002

NOx Emissions PM-10 Emissions
As percent of:     As percent of:
Mode Tons Percent All Mobile Sources All Sources   Tons Percent All Mobile Sources All Sources
Heavy-duty Vehicles 3,782,000 66.8% 33.0% 17.9%   120,000 64.7% 23.3% 0.5%
Freight Railroads 857,200 15.1% 7.5% 4.1%   21,300 11.5% 4.1% 0.1%
Marine Vessels 1,011,000 17.9% 8.8% 4.8%   44,000 23.7% 8.5% 0.2%
Air Freight 8,200 0.1% 0.1% 0.0%   300 0.2% 0.1% 0.0%
Total 5,658,400 100% 49.4% 26.8%   185,600 100% 36.0% 0.8%

Source: U.S. EPA, National Emission Inventory; total mobile source emissions and total emissions obtained from state air quality agencies. Freight railroad emissions estimated as 96.4% of total railroad NOx emissions and 96.7% of total railroad PM-10 emissions, based on passenger locomotive fraction in U.S. EPA, Locomotive Emissions Standards, Regulatory Support Document, April 1998; Air freight emissions estimated as 10.1% of total aircraft emissions, based on air estimated aircraft departures attributable to air freight, as described in report text.

The strict new EPA emission standards for heavy-duty trucks and off-road equipment (such as port cargo handling equipment) will dramatically reduce NOx and PM emissions from these sources starting in 2007. Similar strict standards are expected to be adopted for locomotives and U.S.-flagged commercial marine vessels, but slow fleet turnover means that the full impact of these standards will not be felt for several decades. As a result of the EPA standards, emissions from freight transportation are generally expected to decline over the next several decades, although emissions from some modes will decline more rapidly than others. By 2020, the commercial marine and rail sectors will account for a much larger share of freight NOx and PM-10 emissions than they do currently.

Summary of Regional Freight Transportation Emissions

Freight is also a major source of NOx and PM-10 emissions at the regional level. Among the six regions included in this study, emissions are greatest in magnitude in Los Angeles, followed by Chicago and Detroit. Trucking dominates urban freight movement and related emissions. Heavy-duty trucks are responsible for more than three-quarters of freight emissions in all six regions, as shown in Table ES-2. In Detroit and Dallas-Fort Worth, trucking accounts for virtually all freight emissions - 97 percent of the freight total in Detroit and 93 percent in Dallas-Fort Worth.

The six regions show considerable diversity in terms of freight emissions from other modes. Freight rail NOx emissions in Chicago are nearly twice that in any other region and make up almost 20 percent of Chicago's total freight emissions. In the other five regions, freight rail accounts for less than 10 percent of the total. Marine freight NOx emissions are greatest in the Los Angeles region, where they account for 14 percent of the freight total, and in Houston, where they account for 17 percent of the total. Air freight emissions are dwarfed by the other modes in all six regions. Air freight NOx emissions are greatest in the Los Angeles region, making up 0.5 percent of the region's freight total.

Table ES-2: Regional NOx Emissions from Freight by Mode, 2002

  Trucking Freight Rail Marine Freight Air Freight Freight Total
Region NOx tons % NOx tons % NOx tons % NOx tons % NOx tons %
Baltimore 29,081 83% 2,655 8% 3,315 9% 26 0.1% 35,078 100%
Chicago 96,291 79% 23,212 19% 2,199 2% 462 0.4% 122,164 100%
Dallas-Ft. Worth 53,718 93% 4,157 7% 0 0% 155 0.3% 58,030 100%
Detroit 98,195 97% 2,106 2% 468 0% 40 0.0% 100,809 100%
Houston 64,590 77% 5,163 6% 14,351 17% 85 0.1% 84,189 100%
Los Angeles 130,341 78% 12,744 8% 22,610 14% 870 0.5% 166,564 100%

Source: Compiled and calculated by ICF Consulting, based primarily on data provided by state and regional air quality agencies, MPOs, and ports; see report text for details.

Freight transportation also contributes significantly to regional PM-10 emissions. Trucking is still the largest contributor, although less dominant than with NOx emissions. Marine freight accounts for a major portion of freight PM-10 emissions in regions with large seaports - 40 percent of the total in Houston, 37 percent in Los Angeles, and 19 percent in Baltimore, as shown in Table ES-3. This contribution in part reflects the high PM emission rates of large marine vessels that burn residual fuel and have little or no emission controls.

Table ES-3: Regional PM-10 Emissions from Freight by Mode, 2002

  Trucking Freight Rail Marine Freight Air Freight Freight Total
Region PM-10 tons % PM-10 tons % PM-10 tons % PM-10 tons % PM-10 tons %
Baltimore 734 74% 71 7% 190 19% 1 0.1% 996 100%
Chicago 2,641 73% 792 22% 173 5% 10 0.3% 3,616 100%
Dallas-Ft. Worth 884 88% 113 11% 0 0% 4 0.4% 1,002 100%
Detroit 2,382 96% 58 2% 27 1% 2 0.1% 2,469 100%
Houston 1,256 54% 141 6% 915 40% 2 0.1% 2,314 100%
Los Angeles 2,210 54% 346 8% 1,521 37% 14 0.3% 4,091 100%

Source: Compiled and calculated by ICF Consulting, based primarily on data provided by state and regional air quality agencies, MPOs, and ports; see report text for details.

In the six study regions, total freight emissions account for 40 to 52 percent of all mobile source NOx emissions and 29 to 39 percent of all NOx emissions, as shown in Table ES-4. These regional percentages are significantly higher than the national freight share of NOx emissions (26.8 percent).

Freight accounts for 22 to 47 percent of PM-10 emissions from mobile sources in the study regions. Compared to emissions from all sources, freight accounts for 1.0 to 5.8 percent of regional PM-10 emissions. Again, this is higher than the national freight share (0.8 percent). Freight accounts for the largest share of total PM-10 emissions in the Chicago region, which likely reflects the intensive railroad activity there. Note, however, that the vast majority of PM-10 emissions come from agricultural fields, wildfires, and fugitive dust. The total PM-10 emissions in the six regions, and the portions attributable to freight, therefore, depend heavily on the amount of undeveloped land within the nonattainment boundaries. Note also that the PM emissions from combustion sources like diesel engines are a greater health concern than the coarse particulates from sources like fugitive road dust. Current emission inventories do not provide an accurate estimate of fine particulates, so it is difficult to assess the freight sector contribution to these emissions.

Table ES-4: Total Regional NOx and PM-10 Emissions from Freight, 2002

NOx Emissions from Freight

PM-10 Emissions from Freight


As a percent of:


As a percent of:



All Mobile Sources

All Sources


All Mobile Sources

All Sources















Dallas-Ft. Worth





















Los Angeles







Note: total emissions data were not available for Baltimore.
Source: Compiled and calculated by ICF Consulting, based primarily on data provided by state and regional air quality agencies, MPOs, and ports; see report text for details.

Mitigation Strategies

Strategies to reduce emissions from freight transportation can be grouped in two major categories:

Technological strategies focused on pollutant emission reductions are often summarized as the “Five Rs” - Retrofit, Repower, Refuel (with alternative fuels), Replace, and Repair/Rebuild. A retrofit typically involves the addition of an after-treatment device to remove emissions from the engine exhaust. Repowering involves replacing an existing engine with a new engine. Alternative fuels include those that require little or no modification to the engine (such as emulsified diesel or biodiesel) and those that require engine conversion or replacement (such as natural gas). Replacement involves retiring older, higher polluting equipment from service to be replaced with newer equipment that meets more stringent emission standards. Repairing and rebuilding offer the opportunity to reduce freight emissions during regular engine service intervals through routine maintenance or major engine overhauls.

In addition to the “Five Rs” strategies described above, technological strategies that improve fuel economy typically have the added benefit of reducing emissions. Table ES-5 lists some examples of technological options for improving the fuel efficiency of trucks, locomotives, ships, and aircraft.

Table ES-5: Technological Strategies for Improving Freight Fuel Efficiency

Trucking Rail Marine Air
Fuel efficient lubricants Tare weight reduction Larger vessels Aerodynamic improvements
Tare weight reduction Low-friction bearings Improved hull design Lighter weight materials
Aerodynamic improvements Steerable rail car trucks   More efficient engines
Reduced tire rolling resistance Improved track lubricants    

Operational strategies change the way that trucks, locomotives, ships, and aircraft operate, resulting in fewer pollutant emissions. Many of these strategies, though not all, reduce fuel use and result in lower operating costs for the equipment owner. Table ES-6 summarizes some operational strategies that can reduce emissions from freight transportation.

Table ES-6: Operational Strategies for Reducing Freight Fuel Use and Emissions

Trucking Rail Marine Air
Reduced overnight idling Reduced switchyard idling Cold ironing (electrification) Increased load factors
Reduced pick-up/drop-off idling Reduced line haul speeds Reduced port equipment idling Reduced vertical separation minimums
Port access improvements Reduced empty mileage Reduced hotelling time Reduced use of aircraft APUs
Reduced highway speeds Double tracking Reduced vessel speeds Improved runway efficiency
Arterial signal synchronization Train clearance improvement Use of larger ships Use of continuous descent approach
Grade crossing separation Elimination of circuitous routings Hull cleaning Electrification of ground support equipment
Driver training
Reduced empty mileage

Reducing idling is one of the most promising opportunities to reduce freight emissions. For trucks, overnight idling can be reduced through the use of auxiliary power units (APUs) or truck stop electrification, as well as driver training and incentive programs. Switch yard locomotives can be installed with APUs or automatic shut-down devices to limit freight rail idling. Ships can minimize the use of diesel-powered auxiliary engines while in port through “cold ironing,” which involves retrofitting ocean-going vessels to allow them to receive shore power to meet their energy needs while docked. For aircraft, providing electricity and air conditioning to aircraft directly at the gates reduces the need for aircraft APUs and decreases emissions.

Recommendations for Further Research

In order to more comprehensively consider the emissions effects of freight transportation in the planning and project development process, additional research is needed in a number of areas. In particular, there are some significant shortcomings in the current practices for estimating regional freight emissions:

The other major area for improvement is the understanding of the effects of operational strategies on emissions. Assessing the effect of operational strategies can be difficult because it often requires modeling the performance of an integrated transportation system. A more complete understanding of these effects is needed to support public agencies that are considering investments to improve freight operating efficiency in the name of reducing emissions. Some specific areas for research include:

Public agencies must continue to better integrate freight into the transportation and air quality planning processes and improve their understanding of the linkages between freight transportation and air quality. Through a more integrated approach to planning and better knowledge about freight emissions impacts, agencies can help to ensure the continued efficiency and reliability of the freight system while, at the same time, supporting societal goals related to public health and the environment.

Updated: 5/17/2017
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