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

Emission Mitigation Strategies

As illustrated in the previous section, there is a need to significantly reduce freight transportation emissions in major metropolitan areas. This section describes strategies to reduce emissions from freight transportation. These strategies can be grouped in two major categories:

This section reviews technological and operational emission reduction strategies that apply to one or more of the major freight emissions sources: trucking, railroads, marine vessels, port cargo handling equipment, aircraft, and airport ground support equipment. Selection of any particular strategy depends greatly on the cost effectiveness of the strategy, a complex issue that is not discussed here.

4.1 Technological Strategies

Examples of Retrofit Technologies

Diesel oxidation catalysts use a chemical process to break down pollutants into less harmful components. They have been used for over 20 years and are perhaps the most proven after-treatment device. Diesel oxidation catalysts can reduce emissions of PM by 20 to 50 percent, but do not affect NOx emissions. They work best when combined with ULSD.

Diesel particulate filters collect particulate matter in the exhaust stream. The high temperature of the exhaust heats the ceramic structure and allows the particles inside to break down into less harmful components. These filters can be installed on both new and used vehicles, but they must be used in conjunction with low sulfur diesel. Diesel particulate filters can reduce PM emissions by 50 to 90 percent, but do not affect NOx emissions.

NOx catalysts employ a chemical process to reduce NOx emissions, although these devices have not been tested extensively in off-road applications. Lean NOx catalysts have been shown to reduce NOx emissions by 10 to 20 percent. NOx adsorbers can eliminate more than 70 percent of NOx, but require the use of ULSD.

Selective catalytic reduction (SCR) technology is currently employed at many power plants to chemically reduce NOx emissions to nitrogen and water, but has only recently been adapted to vehicles and other mobile sources. SCR requires a reducing agent (ammonia or urea) to be injected into the exhaust stream. SCR has been shown to reduce NOx emissions by 75 to 90 percent and PM emissions by 20 to 30 percent. An SCR system can be used in conjunction with a diesel particulate filter to achieve much greater particulate reduction.

Technological strategies focused on pollutant emission reductions are often summarized as the “Five Rs” - Retrofit, Repower, Refuel (with alternative fuels), Repair/Rebuild, and Replace.


A retrofit typically involves the addition of an after-treatment device to remove emissions from the engine exhaust. Retrofits can be very effective at reducing emissions - eliminating up to 90 percent of pollutants in some cases. Many of the effective after-treatment devices require use of ultra-low sulfur diesel (ULSD). Some examples of after-treatment devices used for diesel retrofits are summarized in the box to the right.


Repowering involves replacing an existing engine with a new engine. This strategy is most effective for use in equipment with a useful life longer than that of the engine. Repowering provides an opportunity to install a new engine that meets much lower emission standards than the original engine, often in conjunction with fuel economy benefits and lower maintenance costs. Repowering can also include converting diesel-powered equipment (such as port cranes) to electrical power.

Alternative Fuels

A variety of alternative fuels can be used in freight vehicles and equipment. Some require little or no modification to the engine (such as emulsified diesel or biodiesel) while others (such as natural gas) require engine conversion or replacement. The alternative fuels summarized in the box to the right can reduce emissions from many types of diesel engines, although some come at a price of lower fuel efficiency or power.

Examples of Alternative Fuels

Emulsified diesel is a blended mixture of diesel fuel, water, and other additives that reduces emissions of PM and NOx. Emulsified diesel can be used in any diesel engine, but the addition of water reduces the energy content of the fuel, so some reduction in power and fuel economy can be expected. Emulsified diesel has been certified by both EPA and CARB for emission reductions. Expected NOx reductions are in the range of 17 to 20 percent; PM emission reductions range from 17 to 50 percent. Emulsified diesel typically increases VOC emissions.

Biodiesel is a renewable fuel that can be manufactured from new and used vegetable oils and animal fats. Biodiesel is safe and biodegradable and reduces emissions of PM, CO, HC, and air toxics. However, emissions of NOx increase with the concentration of biodiesel in the fuel. Biodiesel is often used as a blend, typically 80 percent petroleum diesel and 20 percent biodiesel (B20).

Natural gas, in the form of compressed natural gas (CNG) or liquefied natural gas (LNG), can be used to power off-road engines. Existing diesel engines can sometimes be converted to run on natural gas, or the existing engine can be replaced with a natural gas engine. There is often a fuel penalty incurred when migrating from traditional diesel fuel. In addition, the use of natural gas raises some challenges with respect to storage and safe handling of the fuel. Because of its fossil fuel base, natural gas is not an effective strategy for reducing GHG emissions.

Propane can also be used to power diesel engines in some applications. Commercial kits are available for retrofitting diesel engines to operate on liquid propane gas (LPG). A number of diesel yard tractors at southern California ports were recently converted to LPG. Compared to unregulated (Tier 0) yard tractors, LPG can reduce NOx and PM emissions by approximately 80 percent. Because of its fossil fuel base, propane is not an effective strategy for reducing GHG emissions.

Ethanol can be blended with diesel to reduce some emissions. Sometimes known as "E-diesel" or "oxydiesel", these blends typically have 10 percent ethanol. Ethanol-diesel blends have not been widely used.

In addition to these fuels, ULSD can help to reduce diesel emissions. As described in Section 2.3, ULSD has less than 15 ppm sulfur, compared to 500 ppm typically used in today's on-road diesel and 3,000 ppm in today's off-road diesel. The primary purpose of ULSD is to enable or improve the performance of after-treatment technologies, such as a particulate filter. Use of ULSD alone (without after-treatment) can reduce PM emissions by 15 to 20 percent compared to higher sulfur diesel.

Hybrid-electric power may soon offer fuel savings and emission reductions in a number of freight applications. For example, many freight railroads are currently experimenting with hybrid switcher locomotives, such as the “Green Goat” manufactured by RailPower Technologies Corporation. The Green Goat relies on battery power to run electric traction motors on the axles. The lead acid batteries are charged by a small onboard diesel-powered generator and microturbine. The reduced reliance on diesel fuel allows for a 30 percent reduction in fuel use and up to a 90 reduction in NOx emissions, compared to a conventional switcher locomotive.


Selectively replacing older freight equipment can sometimes be the most cost-effective way to reduce the emissions of a fleet. In this way, older, higher polluting equipment is retired from service before it would otherwise be retired. Newer equipment that meets more stringent emission standards is purchased to replace the retired equipment, sometimes in conjunction with retrofit devices or alternative fuels. These programs are sometimes called “scrappage” or “fleet renewal” programs. Such programs often include procedures to ensure that the retired equipment is destroyed in order to prevent re-sale and continued use. Fleet owners often benefit from improved fuel economy and performance, as well as lower maintenance costs.


All freight equipment requires periodic maintenance. Routine maintenance and repairs help to ensure that engines operate at maximum performance and emission rates do not exceed the designed standard. Major maintenance intervals provides an opportunity to have the engine rebuilt using more modern, cleaner equipment that provides an immediate emission reduction benefit.

Improving Fuel Efficiency

In addition to the “Five Rs” strategies described above, technological strategies that improve fuel economy typically have the added benefit of reducing emissions. Generally, a reduction in fuel use leads to a commensurate reduction in pollutant emissions. Table 4-1 lists some examples of technological options for improving the fuel efficiency of trucks, locomotives, ships, and aircraft.

Table 4-1: Technological Strategies for Improving Freight Fuel Efficiency





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



4.2 Operational Strategies

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 4-2 summarizes some operational strategies that can reduce emissions from freight transportation.

Table 4-2: 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      

Trucking Operational Strategies

One of the most effective opportunities to reduce truck emissions is to reduce unnecessary idling. Idling is most extensive when trucks are parked at truck stops or other roadside rest areas, often to allow the driver to sleep. Drivers idle for extended periods in order to heat or cool the cab, to run electrical appliances, to keep the engine warm, or simply out of habit. Using a heavy-duty truck engine to provide temperature control or electricity is grossly inefficient and causes unnecessary fuel consumption and pollutant emissions.

A variety of technologies are available that provide cab heating, cooling, and/or electrical supply while consuming far less energy. These include:

Many large truck stops are located on the edge of metropolitan areas, often within the boundaries of an ozone nonattainment area. Thus, idling at these truck stops can contribute significantly to a region's air quality problems. While the amount that trucks idle per night is not well understood, several studies have estimated that long-haul trucks idle approximately six hours per night.51, 52 We estimate that reducing all overnight idling by 50 percent would reduce NOx emissions by 156 tons per year in the Dallas-Fort Worth area and 524 tons per year in the Houston area. There reductions represent 0.3 and 0.8 percent of the on-road heavy-duty vehicle emission inventories in these regions, respectively.

Truck drivers also idle for extended periods when waiting to pick up or drop off a shipment. While a portion of driver wait time may be attributable to carriers building buffers into their schedules to ensure on-time pickup and delivery, the biggest contributing factor appears to be delay caused by shippers and receivers. Shippers can improve scheduling with enhanced communications or logistics software. They can also provide climate-controlled comfort stations at docking facilities and, possibly, couple this with a no-idling policy.

Roadway congestion causes truck delay, idling, and excess emissions. While trucks experience roadway congestion in every urban area, some of the most obvious congested locations are international borders, toll facilities, grade crossings, and port terminal gates. At borders, lengthy immigration and security procedures can contribute to long delays for trucks. The Detroit border crossings, for example, handle more than 5 million commercial trucks per year. Backup times for trucks averaged almost 30 minutes in 2002 and exceeded one hour at busy times on many days.53 Greater use of electronic pre-clearance can help to streamline border operations and reduce congestion. Physical capacity expansion may also be needed at some border crossings.

It is important to also note that the effects of congestion on emissions are sometimes unclear. 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. Many trucking companies have adopted a maximum speed policy for their drivers as a way to save fuel expenses and to promote safety. State and local agencies have also considered highway speed reductions as a way to reduce emissions. For example, the Tennessee Department of Transportation recently agreed to reduce the truck speed limit in Shelby County to 55 miles per hour as a way to help the region attain ozone standards.54

Driving practices can have a large impact on fuel economy. In addition to limiting speed and idling time, drivers can improve fuel economy through their acceleration practices, shifting technique, route choice, use of accessories, and number of stops. Driver training can be provided in-house (at large fleets), through vocational schools, or by outside consultants affiliated with training organizations. An effective program also includes monitoring of driver performance after training and incentives for drivers who reduce fuel consumption. Data from electronic engine monitors can be used by trainers to review detailed operating patterns with drivers and benchmark performance over time. If properly designed and implemented, incentive programs have been found to be very effective at changing driver behavior.

Trucks can also improve efficiency and reduce emissions by reducing empty mileage. When motor carriers cannot arrange for a return shipment, drivers may be forced to pull empty trailers. It is not uncommon to find that empty driving accounts for 20 percent of all mileage for long-haul trucks. Particularly for smaller trucking companies and regional operations, there are opportunities to reduce empty mileage through improved freight logistics. Minimizing empty mileage, as well as other inefficient practices, results in greater fuel productivity (more ton-miles per gallon), which reduces emissions and, at the same time, increases profits for trucking companies.

Rail Operational Strategies

As with trucks, an effective operational strategy to reduce locomotive emissions is to reduce idling. Locomotives may idle for as long as eight hours while cars are switched or while the train waits on a siding for other trains to pass. Idling may also be needed to keep the engine warm in cold weather and to keep accessories from freezing. However, locomotives are often kept idling even when there are no operational reasons to do so. EPA estimates that idling accounts for 60 percent of switch yard locomotive operating time and 12.5 percent of line-haul locomotive operating time.55

In order to reduce idling time, fuel consumption, and pollutant emissions, an APU can be used to provide power when a locomotive is idling. The CSX Corporation has developed an APU that automatically shuts down the main locomotive engine while maintaining all vital main engine systems, such as climate control and heating engine fluids in cold weather. The device is powered by a small diesel engine, and parallels all circulation systems on the locomotive. CSX and International Road and Rail, based in Canada, have formed a joint venture company called EcoTrans Technologies to manufacture and sell the system. EcoTrans estimates that the APU can eliminate 90 percent of switcher idling time. We estimate that retrofitting 50 percent of the switcher locomotives in the Baltimore and Houston regions with APUs, and reducing idling to the extent possible with these devices, would reduce annual NOx emissions by 231 tons and 277 tons respectively. These reductions represent 10 percent and 6 percent of the total annual freight railroad emissions in these regions, respectively.

Locomotives can also be installed with automatic shut-down devices. These devices monitor the locomotive temperature and restart it as necessary to maintain minimum temperatures. Newer locomotives are also equipped with a low idle setting that reduces fuel use and emissions during extended idle periods. Replacing older switch yard locomotives with these newer units can help to reduce the emissions associated with idling.56

Trains can improve fuel efficiency and reduce emissions by operating at lower maximum line-haul speeds. Railroads sometimes take this step on one or more lines in an effort to cope with higher fuel prices. For example, in 2001 BNSF experimented with operating eastbound intermodal trains between New Mexico and Chicago at a maximum speed of 60 miles per hour rather than 70.57 Of course, if railroads lower train speeds to the point where service is inadequate to shippers, they risk diverting traffic to trucks.

Freight rail emissions also can be reduced by improving line-haul efficiency and reducing rail system congestion. However, the interconnected nature of the rail system means that it is much harder to identify and remove the causes of congestion. Rail system congestion can quickly ripple throughout the nation. If one location becomes clogged, locomotives are delayed and unable to meet their next assignments, crews exhaust their Federally-mandated on-duty hours and need to be replaced, and rail cars miss their connections.58 Thus, rail congestion in Arizona or New Mexico can increase emissions in Los Angeles. In this way, the freight rail system has far more in common with the air travel system than with roads, although the nature of the network inefficiencies is different.

There is widespread agreement that the nation's freight rail system is operating at levels of utilization that produce substantial congestion in many places and risk near-gridlock in the future.59 While there are no comprehensive analyses of the national rail system congestion, its effects are evident in a number of ways. For example, Union Pacific has recently been turning away traffic (both bulk and intermodal) and canceling some existing customers' trains in an effort to keep its system fluid.60 The current media are full of accounts of various rail system breakdowns.61 Rail system congestion is also evident in a drop in average train speeds since 1992.62 While speed itself is largely unrelated to locomotive emissions, slower average train speeds generally indicate more idling and starts and stops en-route, which leads to higher emissions.63 The solutions to rail system congestion problems are complex, but clearly the railroad companies' lack of investment capacity has contributed to a decline in net capital stock.

Marine Operational Strategies

Ships can reduce emissions by minimizing the use of diesel-powered auxiliary engines while in port. Ships typically run their auxiliary engines while docked (termed “hotelling”) in order to provide electrical power to the ship for climate control, lighting, cargo refrigeration, on-board cargo handling equipment, and other uses. Hotelling emissions can make up a major portion of total port emissions. For example, hotelling emissions account for 32 percent of all marine vessel NOx emissions at the Port of Houston and nearly 20 percent at the Port of Los Angeles.64, 65

Cold ironing involves retrofitting ocean going vessels to allow them to receive shore power to meet their energy needs while docked at the port, thus allowing them to shut off their auxiliary engines. This strategy is most effective for ports and vessels that have long hotelling times, multiple annual vessel calls, and high auxiliary power needs. The China Shipping terminal at the Port of Los Angeles docked its first commercial container ship using cold ironing in June 2004, and the Port of Seattle plans to implement cold ironing for cruise ships in 2005.66

We estimate that using cold ironing to reduce hotelling emissions for 50 percent of the vessels calling on the ports of Baltimore and Los Angeles would reduce annual NOx emissions at these ports by 567 tons and 808 tons respectively. These values represent 17 and 4 percent of the total annual marine vessel NOx emissions in the Baltimore and Los Angeles regions, respectively.

Reducing ship speed typically reduces emissions. Ships calling on a port travel at cruise speed in open water before entering a port's “reduced speed zone,” as described in Section 3.4. Vessel speed reductions can be promoted by expanding the reduced speed zone further into the cruise region or lowering the specified reduced speed. For example, the Ports of Los Angeles and Long Beach have established a Voluntary Commercial Ship Speed Reduction Program, which urges vessels to travel at or below 12 knots within 20 miles of the coast.

Aircraft Operational Strategies

A number of operational strategies are being explored to improve air traffic management, many of which will result in lower fuel use and emissions. These strategies are commonly referred to as CNS/ATM (communication, navigation, surveillance/air traffic management). They will allow more accurate aircraft approach routes, increase runway efficiency, and reduce aircraft arrival spacing. One example is Reduced Vertical Separation Minimums (RVSM). This involves reducing the vertical separation between aircraft at cruise altitude from 2000 feet to 1000 feet. The effect is an increase in airspace capacity, particularly for long-distance and fuel efficient flights, which allows for greater aircraft scheduling and routing flexibility.67 RVSM became standard in the U.S. in January 2005.

Another operational strategy involves providing electricity and air conditioning to aircraft directly at the gates, which reduces the need for aircraft APUs and decreases emissions. Many large and medium size aircraft use APUs when the main engine is shut down at the airport gate. A report for EPA estimates that, while APU emissions cannot be completely eliminated due to their use during engine startup, APU emissions can be reduced by up to 90 percent.68 While these electrified gates are available at airports across the U.S., some air carriers choose not to use these gates because the time it takes to hook the aircraft up to the system reduces the efficiency of their established operations while cleaning and preparing the aircraft for the next flight.69

A longer term operational strategies for reducing aircraft emissions is the use of continuous descent approach (CDA). In a standard approach to an airport, an airplane is brought down in stages - descending and leveling off several times before landing - with the final level flight segment only 1,000 feet above the airport. Each time an aircraft descends from an intermediate altitude and levels off, thrust must be applied to maintain level flight, which increases emissions relative to a continuous gradual descent. Gradual and continuous descent approach, in which wing flaps and engine thrust are employed differently with the engine operating in idle, or near idle, is not only more fuel-efficient but also quieter. CDA is being implemented at a number of airports in Europe. In the United States, CDA has been successfully tested at Louisville International Airport in partnership with United Parcel Service in October 2002. These tests suggest that up to 500 pounds of fuel could be saved on each flight using CDA.70

Updated: 8/12/2014
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