Freight Movement & Air Quality

Chapter 5: Conclusions and Recommendations

Demand for freight transportation has been rising steadily, and forecasts show continued growth over at least the next several decades. This growing demand is straining portions of the nation's freight system, including some intercity corridors and critical intermodal connections. In an economy where many goods are needed in tightly scheduled manufacturing and distribution systems, deterioration in the reliability of freight travel times can have serious economic repercussions. In response to these challenges, many public agencies are considering freight system investments, including both capacity additions and operational improvements.

As freight becomes more integrated into the transportation planning and programming process, there is a need to consider more comprehensively the effects of freight movement on emissions and air quality. At the same time, increasing concerns about the health effects of diesel exhaust, coupled with the implementation of new air quality standards for ozone and fine particulates, will require many regions across the country to find new ways to control NOx and PM emissions from freight transportation sources. This report attempts to fill a void in the current understanding of the air quality impacts of freight transportation.

Summary of Freight Transportation Emissions

This study shows that freight is a major source of national and regional NOx emissions. At the national level, freight transportation accounts for half of mobile source NOx emissions and 27 percent of NOx emissions from all sources. At the regional level, freight accounts for a similar portion of mobile source NOx emissions (40 to 52 percent in the six study regions) and a larger share of total NOx emissions (29 to 39 percent in the six study regions).

Trucking is the major source of freight NOx emissions, accounting for 67 percent of freight emissions nationally and 77 to 97 percent of the freight emissions total in the six regions. In regions with major seaports (Los Angeles, Houston, and Baltimore), commercial marine freight contributes significantly to NOx emissions, accounting for 9 to 17 percent of the freight emissions total. In Chicago, rail freight accounts for 19 percent of the freight NOx total; in other regions, rail freight accounts for less than 10 percent of NOx emissions from freight. Air freight is responsible for no more than 0.5 percent of total freight NOx emissions in the six study regions and less than 0.1 percent at the national scale.

Freight is also a major source of PM-10 emissions. Freight transportation accounts for 36 percent of mobile source PM-10 emissions nationally, and 22 to 47 percent in the six study regions. Total PM-10 emissions nationally and regionally are dominated by area sources such as agricultural fields, wildfires, and fugitive dust, so freight accounts for only a small portion of total PM-10 emissions (0.8 percent nationally and 1.0 to 5.8 percent at the regional level). Freight contributes a larger share of total fine particulate (PM-2.5) emissions, but emission inventory data does not allow for an accurate assessment of the freight share of these emissions.

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.

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. This study highlights in particular the need for improvements to the standard processes for developing emission inventories for non-road freight modes and the need for a better understanding of the effects of freight operational improvements on emissions.

Improved Emission Inventory Processes

There are a number of shortcomings in the current practices for estimating regional freight emissions:

The process for estimating regional truck emissions typically ignores long-term truck idling. Most state and regional agencies do not have adequate data to properly estimate the extent of truck idling. There is also some uncertainty surrounding the emission factors for extended truck idling and the effects of new emission standards on truck idle emission rates.
Regional truck activity is usually estimated through a process that does not fully account for differences between passenger vehicle and truck behavior. For example, truck speeds are assumed to be the same as passenger vehicle speeds. Forecasts of truck activity are often based on passenger vehicle forecasts. There is a need to improve the modeling of regional truck activity in a way that supports the emissions estimation process. One option would be to undertake this effort as part of FHWA's Freight Model Improvement Program (FMIP).
The standard approach for calculating freight railroad emissions is simplistic and potentially subject to significant errors. Because there is little publicly available data on the operation of private railroads, state and regional air quality agencies must rely on data provided by the railroads themselves. These data can vary widely in their completeness, level of detail, and accuracy. The fuel consumption and emission rates used to estimate locomotive emissions are often outdated or do not reflect local conditions. As a result, estimates of freight railroad emissions are potentially subject to a large degree of error, even though this source can account for nearly 20 percent of total freight NOx emissions in urban areas such as Chicago.
The process for estimating marine vessel emissions varies from sophisticated (in the case of major seaports like Los Angeles, Long Beach, and Houston) to very simplistic and subject to error (in the case of most other ports). Even the sophisticated inventories suffer from marine engine emission factors that are based on limited and outdated data.
Only a handful of regions or ports have developed emission inventories for port cargo handling equipment. In other regions, these sources are lumped together with all off-road equipment for emission inventory purposes, which prevents an accurate assessment of the benefits of mitigation strategies focused on cargo handling equipment.
The process for estimating airport emissions (including aircraft and ground support equipment) is relatively advanced due to the development of the EDMS model and requirements for its use. The model does not distinguish between freight and non-freight activity, but there would typically be no reason to make such a distinction. One significant shortcoming for airport emissions analysis is the lack of sufficient PM emissions data to characterize the current aircraft fleet. FAA is currently working to collect data on aircraft engine PM emissions, which will allow enhancement of the EDMS model to estimate aircraft PM emissions.
The emission factors and methodologies for estimating emissions of fine particulates (PM-2.5) and toxic air contaminants are less robust than for other criteria pollutants. While this study focused almost exclusively on NOx and PM-10 emissions, there are growing concerns about health effects of toxic air emissions and PM-2.5 emissions from diesel engines. There is a need to improve the emission factors for these sources.

Improved Understanding of the Effects of Operational Strategies

The other major area for improvement is the understanding of the effects of operational strategies on emissions. Nearly all technology-oriented strategies to reduce emissions have been the subject of at least some research, and some have been studied extensively. Many technology-oriented strategies are relatively easy to assess because one can analyze the impact on a single vehicle or piece of equipment and then apply this impact to the entire affected population of vehicles/equipment. Assessing the effect of operational strategies on emissions, however, can be more 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:

The effects of changes in roadway congestion on emissions are sometimes unclear or are not properly captured in the tools for estimating emissions. Generally, congested roadway conditions increase emissions because they cause idling and more frequent short bursts of acceleration, when per-mile emission rates are higher than at free flow speeds. However, at steady state speeds over 20 mph, emission rates tend to increase with speed. Per mile emissions of NOx, for example, are almost twice as high at 65 mph than at 20 mph. The MOBILE model uses an average speed for each roadway link. Thus, congestion can sometimes result in lower modeled emissions on certain roadway segments because the average speed of the roadway has been reduced. There is a need to better understand how highway improvements that reduce congestion affect emissions.
In freight systems that are highly integrated, such as railroads or aviation, the emissions effects of congestion are often difficult to assess. Congestion in one location can cause delays to ripple throughout the system, so an increase in emissions might occur far from the bottleneck that triggered it. Research is needed to better understand how changes in freight congestion affect emissions in these conditions.
Significant opportunities exist to reduce freight-related emissions by reducing unnecessary idling. Control strategies include the use of truck stop electrification, APUs for locomotives, cold ironing for ships, and electrified gate equipment for aircraft. Yet there is often a poor understanding of the extent of idling, particularly the extent of idling that can be eliminated through control strategies. More research is needed to assess how operational and technology-oriented strategies can be applied most effectively to reduce idling associated with freight movement.
Better logistics practices can improve the productivity of freight movement, resulting in less fuel consumption and emissions per ton-mile. Freight carriers can achieve better productivity by, for example, reducing empty mileage, increasing load factors, or eliminating circuitous routing. These strategies offer “win-win” solutions in that they increase carrier profitability while minimizing environmental impacts. There is little information on the effects of these types of strategies on emissions.

The growth in freight transportation demand and limited system capacity will create tremendous challenges for transportation planners and decision makers in the coming years. As a result of increases in freight movement, slow fleet turnover, and challenges associated with controlling freight emissions, freight will continue to account for a significant portion of pollutant emissions for many years, both nationally and regionally. Freight has historically been an afterthought in the transportation and air quality planning processes. Public agencies must continue to better integrate freight into these 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.