Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration
Office of Planning, Environment, & Realty (HEP)

Assessing the Effects of Freight Movement on Air Quality at the National and Regional Level

National Freight Transportation Trends and Emissions

This section provides an overview of national-level freight transportation activity and associated emissions. Section 2.1 presents the current volume of freight transportation by mode, reviews recent trends, and discusses primary data sources. Section 2.2 discusses forecasts of freight growth developed by several different organizations and synthesizes these to present plausible freight growth rates by mode. Section 2.3 discusses emissions regulations that affect the freight sector and how emission rates for trucks, locomotives, marine vessels, and aircraft may change over the next 20 years. Section 2.4 presents current and future estimates of national-level freight emissions by mode.

2.1 Freight Transportation Activity and Trends

Although the U.S. economy is becoming more service-oriented, demand for freight transportation has been rising steadily, and forecasts show continued growth at least over the next several decades. In 2001, the Bureau of Transportation Statistics reports that more than 3.18 trillion ton-miles of freight were moved over the nation's domestic transportation system, up almost 22 percent from the 2.61 trillion ton-miles of freight moved in 1990, an annual growth rate of 2.0 percent.14

National Freight Mode Shares

During the period from 1990 to 2001, trucking and rail continued to capture a larger portion of the domestic freight market as measured in ton-miles. As shown in Figure 2-1, trucking market share has grown from 28 percent in 1990 to 33 percent in 2001. Similarly, rail ton-mile market share has continued to grow and represents the largest portion of the inter-city freight market at 47 percent, up from 40 percent in 1990. Trends for domestic waterborne freight have followed an opposite path, with modal share declining during this period from 32 percent of ton-miles in 1990 to 20 percent in 2001. (As discussed later in this section, waterborne imports have grown rapidly.) Air freight has increased in mode share over this eleven year period, from 0.3 percent to 0.4 percent, but still represents a small fraction of overall freight ton-miles.

Figure 2-1: Modal Share of Domestic Ton-Miles, 1990 and 2001

Pie charts for 1990 and 2001 that illustrate the distribution of domestic ton miles by air, intercity truck, waterborne, and class 1 rail freight.

Source: Bureau of Transportation Statistics, National Transportation Statistics 2004.

Figure 2-2 shows the modal share of freight shipment tonnage, including international air freight and waterborne shipments. Trucking moves approximately two-thirds of freight tonnage nationally. Marine vessels move nearly 20 percent of freight tonnage, a large portion of that international shipments. Rail moves 15 percent of U.S. freight tonnage, and aircraft move only 0.1 percent.

Figure 2-2: Modal Share of Freight Tonnage, 2002

Source: Bureau of Transportation Statistics, National Transportation Statistics 2004.

Pie charts for 2002 that illustrates the distribution of freight tonnage by mode (marine vessel, air, trucking, and rail).

In value terms, trucking is by far the dominant domestic freight mode. Figure 2-3 shows the value of 2002 freight shipments by mode. Trucking accounts for three-quarters of freight shipment value, followed by parcel, postal, and courier shipments (12 percent), which often move by truck or a combination of air and truck.

Figure 2-3: Domestic Freight Shipment Value by Mode, 2002

Pie charts for 2002 that illustrates the value of domestic freight tonnage by mode (waterborne, air, truck, rail, parcel/U.S. Postal Service, or courier, or Other).

Source: Bureau of Transportation Statistics and U.S. Census Bureau, 2002 Commodity Flow Survey, Preliminary United States Data, December 2003.

Figure 2-4 depicts trends in domestic ton-miles for the four primary freight modes. This figure illustrates rising volumes for the intercity truck and rail modes, and declines in domestic waterborne freight. Air freight, which appears flat in Figure 2-4 due to the scale of the graph, has actually increased 65 percent since 1990.

Figure 2-4: Domestic Freight Ton-Miles, 1990 - 2003

Chart that shows trends for domestic ton miles from 1990 to 2003 for four modes: Intercity truck, class 1 rail, domestic waterborne freight, and domestic air freight.

Source: Bureau of Transportation Statistics, National Transportation Statistics 2004. Intercity truck data not available for 2002 and 2003.

Trucking Activity

The standard measure of trucking activity, and the measure used to assess air quality effects, is vehicle miles traveled (VMT). Nearly all national VMT estimates are derived from the Highway Performance Monitoring System (HPMS), a national data collection and reporting system administered by FHWA in cooperation with state transportation departments. HPMS contains information on the mileage, usage and capacity of various roadway functional types. FHWA processes HPMS data and, using other sources such as the Vehicle Inventory and Use Survey (VIUS), reports national VMT in the Highway Statistics series. These VMT figures are broken down by:

Table 2-1 shows data on VMT by vehicle type derived from HPMS data. The data show that rural roads tend to carry a much higher percentage of trucks, particularly rural Interstates, where nearly 20 percent of VMT is derived from freight trucks.

Table 2-1: Truck VMT and Total VMT by Roadway Type, 2002

VMT (millions)
Roadway Type Single-Unit Trucksa Combination Trucks Total Trucks Total Vehicles Truck VMT as Percent of Total
Interstate Rural
8,745 45,633 54,378 279,962 19%
Other Arterial Rural
14,606 27,818 42,424 433,805 10%
Other Rural
14,963 14,090 29,053 414,393 7%
Interstate Urban
9,106 23,887 32,993 408,618 8%
Other Urban
28,467 27,215 55,682 1,318,978 4%
75,887 138,643 214,530 2,855,756 8%

Note a: Includes only two-axle six-tire vehicles and single-unit trucks with three or more axles.
Source: Federal Highway Administration, Highway Statistics.

Railroad Freight Activity

Freight railroad activity in the U.S. is dominated by the five U.S. and two Canadian Class I railroads.15 Class I railroads carry more than 90 percent of U.S. railroad ton-miles, consuming more than 94 percent of railroad fuel. The Association of American Railroads (AAR) reports various operating statistics for the Class I railroads, including fuel consumption, tons carried, ton-miles, length of haul, and carloads, in its annual Railroad Facts. Figure 2-5 shows recent trends in railroad traffic. The Class I railroads move nearly 29 million carloads annually, up 38 percent since 1990. Intermodal traffic is growing more rapidly. By 2003, the railroads moved nearly 10 million intermodal trailers and containers, up 60 percent since 1990.

Figure 2-5: Railroad Carloads and Intermodal Traffic, 1990 - 2003

Chart that shows trends for railroad carloads and intermodal trailers and containers from 1990 to 2003.

Source: American Association of Railroads, Railroad Facts 2004.

There is limited data available on the operations of Class II and III railroads (regional and short-line carriers). EPA has estimated that the Class II and III railroads consume approximately 6 percent of the fuel used in freight movement by rail, based on information provided by the American Short Line and Regional Railroad Association.16

Waterborne Freight Activity

Waterborne freight statistics are collected and published annually by the U.S. Army Corps of Engineers in the Waterborne Commerce of the United States series. Data include tons shipped, ton-miles, and average length of haul. Detailed port and commodity information is also available. Table 2-2 shows U.S. waterborne freight tonnage, both foreign and domestic. In total, 2.4 billion tons of freight move by water annually. Foreign trade accounts for 58 percent of waterborne tonnage, with import tonnage nearly 2.7 times more than export tonnage. Domestic waterborne tonnage is primarily inland movements (rivers and canals), with smaller amounts moving along the coasts, in the Great Lakes, and within ports.

Table 2-2: U.S. Waterborne Freight Tonnage, 2003 (millions of tons)

Foreign   Domestic   Total
Imports Exports Sub-Total   Inland Coastal Great Lakes Intra-port Intra-territory Sub-Total  


1,005 373 1,378   610 223 90 87 6 1,016   2,394
42% 16% 58%   25% 9% 4% 4% 0% 42%   100%

Source: U.S. Army Corps of Engineers, Waterborne Commerce of the United States.

As shown previously in Figure 2-4, domestic waterborne freight ton-miles are declining. This can be a misleading indicator for the waterborne sector, however, because more than half of U.S. waterborne freight tonnage is international. On a tonnage basis, waterborne freight has been increasing due to the rapid growth in U.S. imports. While domestic waterborne tonnage fell 9 percent between 1990 and 2003 and U.S. waterborne export tonnage fell by 15 percent, waterborne imports grew by 67 percent over that period. As a result, total waterborne freight tonnage has actually increased since 1990 by approximately 11 percent. Figure 2-6 illustrates these trends.

Figure 2-6: Waterborne Freight Tonnage, 1990 - 2003

Chart that shows trends for waterborne freight tonnage (foreign total, foreign imports, foreign exports, and domestic) from 1990 to 2003.

Source: U.S. Army Corps of Engineers, Waterborne Commerce of the United States.

The American Association of Port Authorities (AAPA) collects and reports activity data from its members, which include all major U.S. deep sea ports, including data on container imports and exports. Shipping containers can be 20, 40, or 45 feet long. So to allow for comparisons of container movement, a standardized measure of twenty-foot equivalent units (TEUs) is used. The number of containers handled at U.S. ports is growing rapidly; between 1995 and 2001, the number of loaded containers moving through the top 10 U.S. ports grew by 47 percent, or 6.6 percent annually. Table 2-3 shows the total TEUs at all major U.S. ports. Nearly one-quarter of all container moves through ports involve empty containers.

Table 2-3: U.S. Port Container Traffic, 2002

Loaded TEUs Empty Total
Inbound Outbound Sub-total TEUs TEUs
8,815,397 22,886,370 7,462,851 30,833,171

Source: American Association of Port Authorities

Air Freight Activity

Air freight is transported in dedicated cargo aircraft and in cargo space of passenger aircraft (belly cargo). All large domestic air carriers report operating annual statistics to the Federal Aviation Administration (FAA) by filing “Form 41,” which includes information on revenue passenger miles, revenue ton miles, and fuel consumption. Figure 2-7 shows recent trends in air freight revenue ton-miles by U.S. carriers (domestic and international service). In 1994, passenger and all cargo carriers handled approximately equal amounts of air freight. Since that time, air freight on all cargo carriers has grown 64 percent, while air freight on passenger carriers has remained nearly constant. This is in part a reflection of the trend toward improving passenger load factors, leaving less capacity for freight. Looking at just domestic flights, air freight handled by passenger carriers has actually declined 28 percent since 1994.

Figure 2-7: Air Freight Revenue Ton-Miles by Carrier Type, 1994 - 2002

Chart that shows trends for air freight revenue ton miles (all cargo carriers versus passenger carriers) from 1994 to 2003.

Source: FAA Aerospace Forecasts, various years

Aircraft emissions are typically calculated based on the number of take-offs and landings. Only the aircraft emissions that occur below 3,000 feet are considered to affect ground level air pollution. For this reason, national air cargo ton-miles or fuel use are not appropriate indicators of the contribution of aircraft to air quality problems.

FAA Form 41 data indicate the number of annual passenger and all-cargo aircraft departures, as well as data on the cargo tonnage and number of passengers. Because passenger aircraft carry both passengers and freight, in order to estimate national aircraft emissions attributable to freight alone, it is necessary to apportion passenger aircraft departures into a passenger and freight component. To do this, we estimated the tonnage of the passengers and the tonnage of the freight on every commercial passenger aircraft departure and used these figures to estimate tonnage-weighted departures attributable to passenger and freight activity.17 These results are shown in Table 2-4. Using this process, air freight can be estimated to account for 10.1 percent of total U.S. aircraft departures in 2002 (7.6 percent due to all cargo aircraft and 2.5 percent due to the freight component of passenger aircraft).

Table 2-4: Aircraft Departures Attributable to Freight, 2002

Air Cargo Aircraft   Passenger Aircraft   Total Aircraft




Passenger Activity Freight Activity





Percent   Tonnage-Weighted Departures Percent Tonnage-Weighted Departures Percent   Departures Percent

Source: Based on Bureau of Transportation Statistics, Air Carrier Statistics (Form 41Traffic).

Fuel Consumption

Fuel use is generally proportional to emissions of greenhouse gases. While freight trucks, locomotives, marine vessels, and aircraft are becoming more fuel-efficient over time, growth in freight activity has in some cases outpaced these efficiency improvements. Consequently, freight fuel use has been increasing in the trucking and rail sectors. Figure 2-8 illustrates these trends in fuel consumption. (Note that most commercial aircraft fuel use in this figure is due to passenger movements.)

Figure 2-8: Fuel Consumption by Domestic Freight Mode, 1990 - 2003

Chart that shows trends for fuel consumption by domestic freight mode (heavy-duty trucks, commercial air, waterborne, and class 1 rail) from 1990 to 2003.

Source: Bureau of Transportation Statistics, National Transportation Statistics 2004 (air, waterborne, rail); Federal Highway Administration, Highway Statistics 2003 (truck).

2.2 Freight Transportation Forecasts

The contribution of freight transportation to air quality problems in future years depends on two major factors - the rate of growth in freight movement and changes in the emissions characteristics of trucks, locomotives, ships, and aircraft. This section discusses freight transportation growth forecasts at a national level. The following section discusses the effects of EPA emission standards on future emission rates.

Several recent studies have developed projections of freight transportation demand by mode. This section reviews three independent forecasts and a fourth that is based on a comparative analysis of available forecasts and historic trends. The sources of these freight forecasts are:

A comparison of the forecasts is shown in Table 2-5. Based on the ICF Consulting results, the most rapid growth is expected to occur in the air freight sector (4 percent annual growth), followed by trucking (2.5 percent) and rail (2 percent). Domestic waterborne freight is expected to remain relatively flat (0.7 percent growth). Note that these figures do not reflect the rapidly growing international waterborne sector nor international air freight.

Table 2-5: Comparison of Domestic Freight Demand Forecasts

  Historic Data   Forecasts (compound annual growth rate)
  (ann. growth)   BTS AASHTO ATA ICF
  (ton-miles)   (ton-miles) (ton-miles) (tons) (ton-miles)
  1990-2000   2000-2025 2000-2020 2002-2014 2000-2020
Truck 3.9%a  
2.6% a
Rail 3.6%  
Water -2.5%  
Air 5.2%  

Note a: Intercity truck only.
Source: Historic data from BTS, National Transportation Statistics 2003; forecasts from sources described in report text.

Applying the ICF Consulting forecasts in Table 2-5 to 2001 freight ton-mileage by mode, we estimate the modal market shares of domestic freight ton-miles in 2020. The results are shown in Figure 2-9, which compares market share in 2001 and 2020. Trucking market share is expected to grow to nearly 37 percent. Rail market share remains mostly unchanged; air freight market share is expected to grow to 0.6 percent of ton-miles.

Figure 2-9: Modal Share of Domestic Ton-Miles, 2001 and 2020 Forecast

Pie charts for 2001 and 2020 (forecast) that show changes in the distribution of domestic ton miles by air, intercity truck, waterborne, and class 1 rail freight.

Source: 2001 data from Bureau of Transportation Statistics, National Transportation Statistics 2003; forecasts calculated by ICF Consulting as described in report text.

2.3 Effects of Emission Standards

Future emissions from freight transportation sources are driven primarily by two major factors. One is the growth in freight transportation activity described in the previous section. The other is the emission standards being adopted by EPA for trucks, locomotives, ships, aircraft, and other off-road equipment. The timing of these regulations and their effects vary significantly by mode. This section reviews emission standards applicable to the major freight modes and discusses their impact on future emissions.

Pollutants of Concern

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). NOx reacts with volatile organic compounds (VOC) to form ground-level ozone, commonly known as smog. Ground-level ozone can trigger a variety of health problems including aggravated asthma, reduced lung capacity, and increased susceptibility to respiratory illnesses like pneumonia and bronchitis. People with respiratory problems are most vulnerable, but even healthy people who are active outdoors, such as construction and port workers, can be affected when ozone levels are high. Ozone also contributes to crop damage, ecosystem damage, and other effects. NOx can also form particulate nitrate, especially in western areas of the country.

Many scientific studies have linked breathing PM to a series of significant health problems, including aggravated asthma, difficult breathing, chronic bronchitis, myocardial infarction (heart attacks), and premature death. Increases in particulate matter levels are associated with increased hospital admissions and emergency room visits for people with heart and lung disease, and increased work and school absenteeism.23 Diesel exhaust is of specific concern, because it is likely to be carcinogenic to humans by inhalation and pose a hazard from non-cancer respiratory effects. In addition to EPA, a number of other agencies (National Institute for Occupational Safety and Health, the International Agency for Research on Cancer, the World Health Organization, California EPA, and the U.S. Department of Health and Human Services) have identified the serious health effects of diesel exhaust. PM is also the major source of haze that reduces visibility, and can cause erosion structures such as monuments and statues.

In this study, we focus on particulate matter less than 10 microns in diameter, or PM-10. There is significant concern about the health effects of particulates less than 2.5 microns in diameter, often called “fine particulates.” The particulate matter generated by fuel combustion (such as diesel engines) tends to be smaller on average than particulate matter caused by sources such as windblown dust, to freight transportation contributes more significantly to PM-2.5 than to PM-10. However, EPA has only recently issued new ambient air quality standards for PM-2.5, and many regions have not yet developed accurate estimates of PM-2.5 emissions. This is in part because there has been less research to support the development of PM-2.5 emission factors.

Freight transportation is also a major source of greenhouse gas (GHG) emissions that contribute to global climate change. By far the most important GHG is carbon dioxide (CO2). Although CO2 emissions are not regulated by the Federal government and there is no air quality standard for CO2, numerous states have developed GHG action plans and emission inventories. Several states have specifically addressed transportation-related CO2 emissions through state energy plans, state environmental regulations, or through the transportation planning process.

Truck Emission Standards

EPA has adopted strict new emission standards for on-road heavy-duty vehicles that take effect beginning in 2007. Under these new standards, both NOx and PM emissions must be ten times lower than current (2004) levels, and the 2007 standards represent a 25-fold reduction compared to emission standards in the early 1990s (see Appendix A for details). Thus, emissions from 2007 model year and later trucks will be dramatically lower than most trucks currently in use today. To meet these standards, truck engine manufacturers will need to use exhaust after-treatment devices for the first time, much like the catalytic converters currently found on automobiles. Note, however, that the emission standards apply only to new vehicles in the year of their manufacture; there are no emission standards that apply to in-use vehicles, other than some state regulations on exhaust smoke opacity.

The emission control devices that will allow engine manufacturers to meet these new standards typically cannot tolerate high sulfur levels in fuel. EPA has adopted companion standards for diesel fuel sulfur levels. Beginning in June 2006, on-road diesel fuel must have no more than 0.15 parts per million (ppm) sulfur (ultra-low sulfur), compared to the current standard of 500 ppm. This ultra-low sulfur diesel (ULSD) will be required for off-road applications (such as locomotives and port cargo handling equipment) by 2010.

Table 2-6 illustrates the effect of these emission standards on the composite fleet average heavy-duty truck emission rates. We developed these emission factors using MOBILE6.2 for urban highway driving in 2002, 2010, and 2020. By 2020, the MOBILE model estimates that nearly all active trucks in the nation's fleet will have met the 2007 standards, so NOx and PM-10 emission rates are much lower than those for today's truck fleet. For example, the 2020 NOx emission factor for combination trucks is 20 times lower than in 2002.

Table 2-6: Fleet Average Heavy Duty Truck Emission Factors, Urban Freeway

    Urban Freeway Emission Factors (grams/mile)
  Year VOC CO NOx PM-10 (total) PM-10 (exhaust only)
Single-Unit Gasoline Truck 2002 1.31 51.39 8.12 0.13 0.11
2010 0.35 12.24 5.60 0.09 0.07
2020 0.12 7.74 2.17 0.047 0.025
Single-Unit Diesel Truck 2002 0.42 2.21 22.69 0.42 0.38
2010 0.28 1.10 8.06 0.17 0.13
2020 0.27 0.28 1.24 0.071 0.032
Combination Diesel Truck 2002 0.43 2.48 25.65 0.41 0.37
2010 0.28 1.14 8.38 0.17 0.13
2020 0.20 0.25 1.28 0.073 0.034

Source: Developed by ICF Consulting using MOBILE6.2 and an average urban highway speed of 52 mph.

Locomotive Emission Standards

In April 1998, EPA finalized emission standards for locomotives, which took effect in 2000 and involve a three-tiered system (see Appendix A). The Tier 0 emission standards apply to locomotives and engines originally manufactured from 1973 through 2001, any time the engine is manufactured or remanufactured. Tier 1 standards apply to original model years between 2002 through 2004. Tier 2 standards apply to original model years of 2005 and later. Tier 1 and 2 locomotives are required to meet the applicable standards at both the time of original manufacture and at each subsequent rebuilding. The standards will result in a 45 percent reduction in NOx emissions for Tier I locomotives and a 59 percent reduction in NOx for Tier II locomotives, compared to baseline values. Hydrocarbon (HC) and PM-10 emissions for locomotives built in 2005 and later must be 40 percent lower.

Table 2-7 shows the fleet average emission factors for all locomotives in 2002, 2010, and 2020. These factors reflect Class I line-haul and switch locomotives, Class II and III locomotives, and passenger locomotives, although the factors are dominated by the Class I locomotives. NOx and PM-10 emission rates are expected to decline, although not as dramatically as heavy-duty truck emission rates will decline. Between 2002 and 2020, locomotive NOx emission factors will decline by 44 percent and PM-10 factors will decline by 28 percent.

Table 2-7: Fleet Average Locomotive Emission Factors

  Emission Factors (grams/gallon)
Year HC CO NOx PM-10
2002 10.7 27.4 249.4 6.8
2010 9.2 27.4 163.7 5.7
2020 8.0 27.4 140.8 4.9

Source: U.S. EPA, Locomotive Emissions Standards, Regulatory Support Document, April 1998.

In 2004, EPA announced its intent to propose more stringent emission standards for new locomotive diesel engines. The new standards are expected to be modeled after the 2007/2010 highway regulations and Tier 4 non-road diesel engine regulations (described below), with an emphasis on achieving large reductions in emissions of PM and air toxics through the use of advanced emission control technology.

Marine Vessel Emission Standards

For regulatory purposes, commercial marine engines are classified as Category 1, 2, or 3, based on size. Category 1 marine diesel engines have rated power of at least 50 horsepower and a per-cylinder displacement of less than 5 liters, similar to land-based non-road engines used in construction and farm equipment. Category 2 marine diesel engines have per-cylinder displacements of between 5 liters and 30 liters and are most similar to those engines found in land-based locomotives. Category 1 and 2 engines are used as propulsion engines (i.e., the engine that moves the vessel through the water) or as auxiliary engines to provide on-board electricity. Category 3 marine diesel engines have per-cylinder displacements of at least 30 liters and are used to propel container ships, tankers, bulk carriers, and cruise ships. Most of these engines are installed on ocean-going vessels, though a few are found on ships in the Great Lakes. Category 1 and 2 engines burn distillate diesel fuel, which is similar to non-road diesel. Category 3 engines burn residual fuel, a by-product of distilling crude oil with a high viscosity and density.

EPA established the first emission standards for these engines in 2000 to take effect between 2004 and 2007. The standards require relatively modest reductions in NOx, CO, and PM (see Appendix A). By 2020, these standards are expected to reduce commercial marine NOx emissions by 21 percent, relative to uncontrolled levels, and reduce PM-10 emissions by 11 percent.

In May 2004, EPA announced its intent to propose more stringent emission standards for all new commercial, recreational, and auxiliary marine diesel engines, except Category 3 engines. Like the new standards planned for locomotives, the new marine standards are expected to be modeled after the 2007/2010 highway and Tier 4 non-road diesel engine programs and will result in the use of advanced emission control technology. It is important to note that EPA standards apply only to U.S.-flagged vessels. While the vast majority of Category 1 and 2 engines in U.S. waters are U.S.-flagged, most Category 3 vessels are foreign-flagged and thus not subject to EPA regulations.

The International Maritime Organization (IMO) leads the development of international regulations for ships. The IMO adopted Annex VI of the International Convention on the Prevention of Pollution from Ships (MARPOL) in 1997 to set NOx emissions standards for ships. MARPOL Annex VI will come into force in May 2005, and at that time, any country that has ratified the treaty can enforce the NOx emission standards for any ships in its waters. It applies to engines on ships constructed on or after January 1, 2000. The U.S. Senate has not ratified MARPOL Annex VI. If the U.S. Senate ratifies MARPOL Annex VI, then it can be enforced against any foreign-flagged ship that visits a U.S. port, whether or not the flag state of the ship has ratified the treaty. Until Annex VI is ratified, however, only a small fraction of Category 3 marine engines in U.S. waters are subject to emission regulations.

Off-Road Equipment Emission Standards

EPA issues separate emission standards for off-road diesel engines, a category that includes most of the off-road equipment used to handle cargo at ports as well as some freight-related ground support equipment at airports. These regulations continue to be phased in under a four-tier system, with emission standards based on engine horsepower and equipment model year (see Appendix A). Tier 1, 2, and 3 standards are largely being met by enhanced engine design and manufacturing improvements, requiring little or no exhaust after-treatment, and do not address fuels. The Tier 4 standards require dramatic reductions in NOx and PM emissions, akin to the emission reductions required by 2007 standards for on-road heavy-duty diesel trucks. The non-road NOx and PM standards under Tier 4 are approximately ten times lower than the Tier 3 standards for most engines. They will be phased in between 2008 and 2015. To comply with this rule, engine manufacturers will need to produce engines with advanced emission control technologies similar to those that will be used for on-road trucks.

This ruling also requires fuel producers to reduce the sulfur content of diesel fuel used in non-road engines to 15 ppm (ULSD) by 2010. Reducing the level of sulfur in diesel fuel is necessary to prevent damage to the emission control systems. Use of ULSD in locomotives and commercial marine diesel engines (most Category 1 and 2 engines) will further reduce PM-10 emissions beyond the effects of the standards described above.

Aircraft Emission Standards

EPA works with FAA to regulate aircraft emissions, as well as the International Civil Aviation Organization (ICAO), an international body that typically leads the development of aircraft emission standards. In 1997, EPA aligned the U.S. aircraft emissions standards and test procedures with those prescribed by ICAO, which apply to commercial aircraft engines with rated thrust greater than 26.7 kilonewtons (kN) and cover NOx and CO. In 2003, EPA announced its intent to adopt the revised ICAO standards for engine NOx emissions, which require NOx emissions to be reduced by an additional 16 percent. These new standards affected new engine certifications as of December 31, 2003; EPA expects the regulations promulgating the new standards to be in place by June 2005. Furthermore ICAO has adopted a further increase in stringency of the NOx emissions standards, which will affect new engine certifications as of December 31, 2007 and require an additional 12 percent reduction in NOx emissions.

Commercial jet aircraft have service lives of 25 to 40 years, so it can take decades for a major technological improvement to appear in a majority of the commercial fleet.24 Moreover, aircraft manufactured in the 1990s typically have higher NOx emissions than aircraft from the 1970s and 1980s because noise and fuel consumption reduction technologies employed in the 1990s come at the expense of increased NOx emissions. Thus, until aircraft that meet the ICAO standards begin to dominate the in-use fleet, fleet average emission rates for NOx are expected to remain constant or increase slightly as older aircraft are retired.

Table 2-8 shows an estimate of aircraft emission rates for the global average fleet in 2002 and 2015. These global figures are likely to be representative of the U.S. fleet because aircraft are fairly well integrated globally and U.S. aircraft make up a major share of the global fleet. These emission rates illustrate the expected increase in NOx emissions per unit of fuel consumption, although emission rates for other pollutants will decline. Comparable composite emission rates that reflect the latest ICAO standards are not available, but over time the new standards are expected to reverse the trend toward increasing NOx emission rates.

Table 2-8: Commercial Aircraft Emission Rates, Global Average

  Emission Rates (grams/kg fuel)
2002 1.5 5.3 13.2 0.6
2015 0.7 4.4 14.1 0.4

Source: Sutkus, Donald J., Jr., Steven L. Baughcum, and Douglas P. DuBois, Commercial Aircraft Emission Scenario for 2020: Database Development and Analysis, Prepared for NASA, Prepared by Boeing Commercial Airplane Group, NASA/CR—2003-212331, May 2003.


Figure 2-10 shows the effects of recent EPA emission and fuel standards on heavy-duty vehicle (HDV), locomotive, and commercial marine NOx and PM-10 emissions.25 This figure shows the percent change in emissions in 2010 and 2020 compared to a status quo baseline, as calculated by EPA. The status quo baseline reflects the expected growth in truck, rail, and marine vessel activity, but without any change in emission standards. In the case of HDVs, the baseline reflects the 1998 emission standards; in the case of locomotives and commercial marine vessels, the baseline represents uncontrolled emissions.

Figure 2-10: Change in National NOx and PM-10 Emissions from Baseline, by Mode

Bar chart that compares predicted NOx and PM-10 emissions from HDV, marine, and locomotive modes for the years 2010 and 2002.

Source: U.S. EPA, Regulatory Impact Analysis: Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements, EPA420-R-00-026, December 2000; U.S. EPA, Final Regulatory Impact Analysis: Control of Emissions from Marine Diesel Engines, EPA420-R-99-026, November 1999; U.S. EPA, Locomotive Emissions Standards, Regulatory Support Document, April 1998; U.S. EPA, Regulatory Impact Analysis: Control of Emissions of Air Pollution from Nonroad Diesel Engines and Fuel, EPA420-R-04-007, May 2004.

By 2020, NOx and PM-10 emissions from heavy-duty trucks will drop 83 and 66 percent, respectively, as a result of the 2004 and 2007 emission standards. Locomotive NOx emissions will drop by 49 percent and PM-10 emissions will drop by 39 percent by 2020. The recently issued commercial marine emission standards will have less impact. By 2020, the standards will reduce commercial marine NOx and PM-10 emissions by 21 percent and 15 percent, respectively, compared to uncontrolled levels. Note that this figure does not reflect likely new emission standards for locomotives and commercial marine engines, which EPA has announced its intent to adopt. However, because of the long life of locomotive and marine engines, these standards are unlikely to have a major effect until after 2020.

It is also important to reiterate that most vessels calling on the major U.S. deep sea ports are foreign-flagged and not subject to EPA emission regulations. At these ports, NOx and PM emissions from commercial marine engines are likely to increase over the next several decades.

2.4 National Freight Transportation Emissions

The preceding sections review current and forecast freight activity levels and the influence of emission standards on current and future emission rates. This section illustrates how those two factors are likely to affect total freight emissions by mode between 2002 and 2020.

EPA develops a National Emission Inventory (NEI) that can be used to estimate freight sector emissions nationally and by county. The NEI is developed using a combination of national and local level activity data and input from state and local air agencies. Data from the NEI are used for air dispersion modeling, regional strategy development, regulation setting, air toxics risk assessment, and tracking trends in emissions over time.

Table 2-9 shows U.S. NOx and PM-10 emissions from the four major freight modes for 2002. The NEI does not distinguish between freight and non-freight activity. In the case of railroads, we estimated freight railroad NOx emissions as 96.4 percent of total railroad NOx emissions and 96.7 percent of total railroad PM-10 emissions, based on the passenger locomotive fraction in EPA's Locomotive Emissions Standards, Regulatory Support Document.26 We estimated air freight emissions as 10.1 percent of total aircraft emissions, based on the estimated fraction of aircraft departures attributable to freight, as presented in Table 2-4. The marine vessel emissions in Table 2-9 reflect a small amount of non-freight activity (e.g., cruise ships and ferries), but we have not attempted to subtract this portion from the total. Note that Table 2-9 does not show emissions from off-road cargo handling equipment at ports or airport ground support equipment. No data are available to estimate these components at the national level, although metropolitan estimates of these emissions are discussed in Section 3.

Table 2-9: 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.

Table 2-9 shows that freight transportation accounts for approximately half of mobile source NOx emissions and 27 percent of all U.S. NOx emissions (anthropogenic sources only). Freight transportation accounts for 36 percent of 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 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 and 12 percent, respectively. Air freight accounts for only 0.1 to 0.2 percent of total freight emissions of NOx and PM-10.

To determine future freight transportation emissions, we reviewed EPA documents supporting recent emission regulations. 27, 28, 29, 30, 31, 32 To assess the impact of new emission standards, EPA has estimated current and future emissions from heavy-duty trucks, locomotives, and commercial marine vessels. Because these documents were developed several years ago, they typically contain a 2002 emissions estimate that is different from EPA's 2002 NEI estimate for that mode. It is inappropriate to compare future emissions from the regulatory support documents with current emissions from the NEI. Therefore, to determine future emissions, we multiplied the 2002 NEI estimate (from Table 2-9) by the percent change in emissions forecast in the EPA regulatory support documents. For example, EPA's Regulatory Support Document for the 1998 locomotive emission standards estimates that 2010 locomotive emissions will be 66 percent of the 2002 emissions. We multiplied 66 percent by the 2002 locomotive emissions in Table 2-9 to estimate 2010 emissions. (Note that as a check on these results, we also developed an independent estimate of current and future trucking activity and emissions. These results and a discussion of the methodology are included as Appendix B.)

No similar estimate of future aircraft emissions has been developed by EPA. We estimated future aircraft emissions by applying two scaling factors to the 2002 emissions shown in Table 2-9. First, we scaled up the 2002 aircraft emissions using a 4 percent annual growth rate to reflect the expected increase in activity, as presented in Section 2.2. Then, we adjusted the future emission estimates based on the change in fleet average emission factors shown in Table 2-8. In the case of NOx emissions, we assumed no change in fleet average emission factors, rather than the slight increase shown in Table 2-8, due to the expected impact of the latest ICAO emission standards. In the case of PM-10, we assumed fleet average emission factors would change in proportion to SO2 emission factors.

Based on the methodology and assumptions outlined above, Table 2-10 shows current and future NOx emissions from freight and the percent change from 2002 levels. These estimates show total freight emissions declining 63 percent by 2020. Truck emissions are estimated to experience the greatest decline (82 percent), followed by freight rail (43 percent). Commercial marine emissions are expected to decline only slightly by 2020 (7 percent), while air freight emissions are expected to increase 51 percent. While air freight emissions are estimated to increase, they represent only 0.6 percent of the total projected 2020 freight transportation NOx emissions. These figures do not show emissions from off-road cargo handling equipment at ports or airport ground support equipment.

Table 2-10: Current and Future Freight Transportation NOx Emissions by Mode

  Heavy-Duty Trucks Freight Rail Commercial Marine Air Freight Freight Total
 Year tons chnge tons chnge tons chnge tons chnge tons chnge
2002 3,782,000   857,200   1,011,000   8,200   5,658,400  
2010 2,186,900 -42% 563,200 -34% 987,200 -2% 10,000 22% 3,747,299 -34%
2020 662,600 -82% 486,400 -43% 938,600 -7% 12,400 51% 2,099,999 -63%

Source: 2002 data from U.S. EPA, National Emission Inventory, adjusted by ICF Consulting to reflect freight as described in report text; 2010 and 2020 estimates calculated by ICF Consulting based primarily on EPA regulatory support documents as described in report text.

Figure 2-11 compares the relative contribution of the modes to total freight NOx emission in 2002, 2010, and 2020. Currently, trucking dominates freight NOx emissions (67 percent of the total), but the trucking share is expected to decline rapidly by 2020 (31 percent of the total). In contrast, commercial marine emissions currently account for only 18 percent of the freight sector total, but are expected to account for 44 percent by 2020. Freight rail NOx emissions are expected to also grow in significance, from 15 percent today to 23 percent by 2020. The share of total freight NOx emissions for air freight is expected to increase 1.0 percentage point by 2020.

Figure 2-11: Freight Transportation NOx Emissions in 2002, 2010, and 2020

Comparison of 2002 freight transportation NOX emissions for four modes (freight rail, commercial marine, heavy-duty trucks, and air freight) with predicted emissions for 2010 and 2020 for the same modes.

Source: 2002 data from U.S. EPA, National Emission Inventory, adjusted by ICF Consulting to reflect freight as described in report text; 2010 and 2020 estimates calculated by ICF Consulting based primarily on EPA regulatory support documents as described in report text.

Table 2-11 shows current and future PM-10 emissions from freight transportation sources and the percent change from 2002 levels, based on the assumptions and methodology outlined above.33 Total PM-10 emissions from freight are expected to decline 50 percent. As with freight NOx emissions, the reduction is led by trucking, which is estimated to drop 71 percent in PM-10 emissions. Freight rail PM-10 emissions are expected to decline by 39 percent. Commercial marine emissions of PM-10 in 2020 are nearly identical to 2002 levels, because growth in marine activity will offset the effect of EPA emission and fuel standards. Air freight emissions of PM-10 are expected to decline by 10 percent. Again, these figures do not show emissions from off-road cargo handling equipment at ports or airport ground support equipment.

Table 2-11: Current and Future Freight Transportation PM-10 Emissions by Mode


Heavy-Duty Trucks Freight Rail Commercial Marine Air Freight Freight Total
Year tons chnge tons chnge tons chnge tons chnge tons chnge
2002 120,000   21,300   44,000   300   185,600  
2010 65,380 -46% 15,730 -26% 42,930 -2% 290 -3% 124,329 -33%
2020 34,760 -71% 12,990 -39% 44,080 0% 270 -10% 92,099 -50%

Source: 2002 data from U.S. EPA, National Emission Inventory, adjusted by ICF Consulting to reflect freight as described in report text; 2010 and 2020 estimates calculated by ICF Consulting based primarily on EPA regulatory support documents as described in report text.

Figure 2-12 compares the relative contribution of the modes to total freight PM-10 emission in 2002, 2010, and 2020. The trend is similar to NOx emissions - the trucking share of the PM-10 total from freight declines from 65 percent today to 38 percent by 2020. During this period, the commercial marine share doubles from 24 percent to 48 of all PM-10 emissions from freight. Little percentage change is seen for PM-10 emissions attributable to the freight rail and air freight sectors.

Figure 2-12: Freight Transportation PM-10 Emissions in 2002, 2010, and 2020

 Comparison of 2002 freight transportation PM-10 emissions for four modes (freight rail, commercial marine, heavy-duty trucks, and air freight) with predicted emissions for 2010 and 2020 for the same modes.

Source: 2002 data from U.S. EPA, National Emission Inventory, adjusted by ICF Consulting to reflect freight as described in report text; 2010 and 2020 estimates calculated by ICF Consulting based primarily on EPA regulatory support documents as described in report text.

Table 2-12 shows greenhouse gas emissions from freight transportation sources. Emissions are presented in terragrams (Tg) of CO2 equivalents.34 Freight trucks account for more than three-quarters of freight-related GHG emissions, followed by marine vessels and freight railroads. Air freight contributes nearly three percent of freight GHG emissions, a much larger fraction than criteria pollutant emissions but still a small portion of the freight total. Overall, freight is responsible for 6.3 percent of all U.S. GHG emissions and one-quarter of GHG emissions from transportation.

Table 2-12: Greenhouse Gas Emissions from Freight Transportation, 2003

  GHG Emissions (Tg CO2 equivalents)
  Percent of:
Mode Emissions Percent All Transportation Sources All Sources
Heavy-duty Trucks 340.7 77.8% 19.2% 4.9%
Freight Railroads 38.2 8.7% 2.2% 0.6%
Marine Vessels 46.5 10.6% 2.6% 0.7%
Air Freight 12.4 2.8% 0.7% 0.2%
Total 437.8 100% 24.7% 6.3%

Note: Does not include marine and aviation bunker fuels (fuel sold in the U.S. for international transportation).
Source: U.S. EPA, Draft Inventory Of U.S. Greenhouse Gas Emissions And Sinks: 1990-2003, February 2005, adjusted by ICF Consulting to reflect freight as described in report text.

Updated: 8/12/2014
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000