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Multi-Pollutant Emissions Benefits of Transportation Strategies-FHWA

2. Summary of Findings

This section provides an overview of findings regarding the impacts of each type of strategy on each of the seven pollutants examined. It is important to note that the primary purpose of this study is to identify the impacts of each strategy on emissions of each pollutant in terms of direction (e.g., positive, negative, uncertain). This document does not address the relative effectiveness or cost-effectiveness of individual strategies. Within each type of strategy, a wide range of impacts might occur, depending on the scope of implementation (e.g., statewide, regional, local), stringency (e.g., mandatory, incentive-based, or voluntary program; level of financial incentive provided, etc.), and demographic and geographic characteristics (e.g., existing mode shares and levels of travel, availability of travel options, land use patterns, etc.). Moreover, the effectiveness of many of these strategies is enhanced (or conversely, may be inhibited) due to combination with other strategies.

Factors Affecting Emissions Impacts

Type of Strategy Effects

Transportation strategies generally affect emissions by having one or more of the following effects:

Strategy effects are summarized below by major type of effect:

Strategies that reduce vehicle travel - A reduction in vehicle travel can occur in several ways, including shifts from driving to other modes (i.e., transit, bicycling, walking), increasing vehicle occupancy, reducing the number of trips made (e.g., through telecommuting), or reducing vehicle trip lengths (e.g., through better land use mixing). Strategies that reduce vehicle miles traveled (assuming no other effects) will reduce emissions of all pollutants. Each mile that a vehicle travels, it emits more pollution, so reducing vehicle travel mileage will reduce emissions of all seven gases.5 However, in conducting emissions analysis, it is important to examine not only the reduction in vehicle miles traveled (VMT), but also the reduction in the number of vehicle trips. During the first portion of a vehicle trip, when the vehicle engine starts cold, the vehicle emits some pollutants at a much higher rate than during the remainder of the trip, since emissions control technology does not operate as efficiently as when the vehicle is warm. Some strategies reduce VMT by shortening vehicle trip lengths but do not reduce the number of vehicle trips. For instance, development of a park-and-ride lot may reduce VMT by encouraging carpools, but the park-and-ride lot generally does not reduce vehicle cold starts, only running emissions, since individuals must drive to the lot in the morning. On the other hand, most bicycle/pedestrian projects reduce vehicle trips entirely, and will eliminate both cold start and running emissions. Consequently, VMT-reducing strategies may result in different percentage reductions in different pollutants, depending on whether or not vehicle trip cold starts are reduced.

In MOBILE 6.2, incremental emissions associated with a cold start only occur for VOCs, NOx, and CO. In general, among the types of vehicles affected, a reduction in VMT that occurs entirely through vehicle trip elimination (such as a bicycle project) will result in a nearly proportional reduction in emissions of all pollutants from light-duty motor vehicles. For instance, reducing light-duty vehicle commute travel by 5 percent due to mode shifts should result in approximately a 5 percent reduction in emissions of all pollutants by light-duty vehicles on work trips, assuming the same emissions factors and not accounting for emissions from the other mode (transit).6 On the other hand, a strategy, like a park-and-ride facility, which reduces vehicle trip lengths but does not eliminate cold starts, will most likely result in a lower percentage reduction in VOCs, NOx, and CO than other pollutants, since the first few miles of the trip produce a higher share of total trip emissions.

Strategies that reduce vehicle idling - Strategies that reduce vehicle idling (assuming constant emissions factors and no other effects that would further impact emissions) will reduce emissions of all pollutants, since some of each pollutant is producing during engine operation even if a motor vehicle is not moving. Specifically, the combustion process results in exhaust emissions of all seven pollutants. Running loss evaporative emissions also occur during idling, as the hot engine and exhaust system vaporizes gasoline, causing additional release of VOCs. Emissions factors during vehicle idle can be generated using MOBILE6.2 In addition, EPA has developed specific guidance and emissions factors for examining long-duration idling; this guidance, however, currently only addresses NOx and PM.7

Strategies that affect vehicle speeds and traffic flow - Strategies that affect vehicle speeds and traffic flow conditions will have different impacts on different pollutants, and may reduce emissions of some pollutants while increasing or having no effect on emissions of others. In MOBILE6.2, emissions rates for VOCs, NOx, and CO vary with vehicle speed. However, in general, MOBILE6.2 emissions rates for PM-10, PM-2.5, SOx, or NH3, do not vary with vehicle speeds. PM emissions are affected slightly due to tire and break wear. Figure 2-1 shows emissions factors by speed for CO, VOCs, NOx, and the four other pollutants (all have less than 0.1 g/mi). As a can been seen here, strategies that result in higher average speeds might reduce VOCs but could increase CO and NOx emissions. Strategies that involve shifting traffic from peak to off-peak periods, therefore, could also increase CO and NOx emissions. The direction of the impact depends on the speeds of traffic without and with the strategy implemented.

Figure 2-1: Emissions Factors by Speed for Light-Duty Vehicles and Trucks, 2006 (arterials)

Figure 2-1 shows emissions factors by speed for light-duty vehicles and trucks for 2006, based on arterials. While other pollutants are not affected speed, the chart shows the variance curves for VOCs, NOx and CO in grams per mile (g/mi) as a factor of miles per hour (mph). CO emissions average over 30 g/mi at 2.5 mph, and decrease to approximately 10 g/mi at 65 mph; VOCs emissions approximate 9 g/mi at 2.5 mph and decrease to less than 0.5 g/mi at a speed of 65 mph; NOx emissions average just over 5 g/mi at 2.5 mph and decrease to under 1 g/mi at 65 mph.

Strategies that alter vehicle fleet characteristics - Strategies that affect vehicle age, fuels, engine technologies, or emission control technologies will have effects that differ by pollutant. Many emission control technologies (catalysts, filters, etc.) and alternative fuels are designed to reduce only selected pollutants. For example, diesel oxidation catalysts (a common retrofit for older trucks and construction equipment) reduce PM emissions but have no effect on NOx emissions. Biodiesel blends have been found to reduce emissions of VOCs, CO, and PM, but to lead to slight increases in NOx; higher percentage biodiesel blends produce larger VOCs, CO, and PM reductions and larger NOx increases. Additionally, it is important to recognize the distinction between national engine and fuel standards that are set by the EPA and strategies that can be carried out at the state or local level to alter fleet characteristics for emissions reductions.

Types of Vehicles Affected and Share of Inventory

In determining which strategies will be most effective at reducing specific pollutants, it is useful to understand what share of the emissions inventory for that pollutant comes from the types of vehicles that are being affected by the strategy. The share of emissions coming from individual sources differs widely among regions, depending on types of industries, amount of freight traffic, and other factors. Using national figures, however, some general patterns are apparent (see Figure 2-2).

Figure 2-2: Contribution of Mobile Sources to U.S. Emissions
From National Emissions Inventory Air Pollutant Emissions Trends Data, 2002
Figure 2-2 shows a graphic representation of mobile source contributions to U.S. emissions, based on the 2002 EPA National Emissions Inventory Air Pollutant Emissions Trends Data. The x-axis displays type of pollutant and the y-axis displays percentage of total contributor of all sources. The amounts displayed are approximately as follows: NOx - 0% road dust, 19% off-highway, 16% highway diesel, 2% heavy-duty gas, 17% light duty gas; CO - 0% road dust, 22% off-highway, less than 1% highway diesel, 2% heavy-duty gas, 52% light duty gas; VOCs - 0% road dust, 16% off-highway, 1% highway diesel, 1% heavy-duty gas, 2% light duty gas; NH3 - 0% road dust, less than 0.1% off-highway, 0.2% highway diesel, 0.1% heavy-duty gas, 7.5% light duty gas; SOx - 0% road dust, 2.7% off-highway, less than 1% highway diesel, less than 0.1% heavy-duty gas, 1% light duty gas; PM-10 - 18% road dust, 1.4% off-highway, 0.5% highway diesel, less than 0.1% heavy-duty gas, 0.4% light duty gas; PM-2.5 - 10% road dust, 4% off-highway, 1.5% highway diesel, 0.1% heavy-duty gas, less than 1% light duty gas.

Summary Impacts of Strategies

A summary of the types of transportation system effects and the general direction of emissions impacts, by strategy, is presented below. These impacts are based on reviews of reference documents and case studies, understanding of EPA's existing emissions models, and professional judgment. It should be noted that some types of effects have not been quantified by EPA; for instance, in MOBILE6.2, PM, SOx, and NH3 emissions are not affected by changes in travel speeds, and EPA guidance does not quantify the impacts of some retrofits on certain pollutants. In these cases, impacts are recorded as no effect or not available.

Impacts are indicated in general terms, and are not intended to represent magnitude. In some instances, reductions or increases of one pollutant may be much larger than for another; the sample strategies provide an indication of the relative magnitude of impacts for each pollutant.

General Emissions Impacts of Transportation Demand Management Strategies

Transportation demand management (TDM) strategies focus on changing travel behavior - trip rates, trip length, travel mode, time-of-day, etc. Most TDM projects and programs reduce emissions by reducing trips and/or vehicle miles traveled (VMT) by personal motor vehicles, or by shifting trips from peak periods to less congested periods. In general, strategies that reduce VMT will reduce emissions of all pollutants. Transportation analysts should be aware that some specific strategies have the potential to increase one or more pollutants, but this would generally not be the case for a program aimed at emissions reductions.8 In addition, strategies that reduce vehicle travel may also have an indirect impact on travel speeds; however, these effects are generally minor and would not be large enough to offset emissions reductions. The table below provides a summary of the general emissions impacts of selected TDM strategies.

Table 2-1: General Emissions Impacts of TDM Strategies

Strategy Category of Primary Effect General Pollutant Effect
Reduce VMT Reduce vehicle trips Shift travel time Reduce idling Change speeds Change vehicle stock or fuels PM-2.5 PM-10 CO NOx VOCs SOx NH3
1. Park-and-Ride Facilities primary effect -         decrease decrease decrease decrease decrease decrease decrease
2. HOV Lanes primary effect primary effect     primary effect   decrease decrease decrease decrease decrease decrease decrease
3. Ridesharing primary effect may be a notable effect, but not in all cases         decrease decrease decrease decrease decrease decrease decrease
4. Vanpools primary effect may be a notable effect, but not in all cases         decrease decrease decrease decrease decrease decrease decrease
5. Bicycle/Pedestrian primary effect primary effect         decrease decrease decrease decrease decrease decrease decrease
6. Transit Service Enhancement primary effect primary effect         Down Arrow with Asterisk Down Arrow with Asterisk decrease Down Arrow with Asterisk decrease Down Arrow with Asterisk decrease
7. Transit Marketing, Information and Amenities primary effect primary effect         decrease decrease decrease decrease decrease decrease decrease
8. Transit Pricing primary effect primary effect         decrease decrease decrease decrease decrease decrease decrease
9. Parking Pricing/ Management primary effect primary effect         decrease decrease decrease decrease decrease decrease decrease
10. Road Pricing primary effect primary effect may be a notable effect, but not in all cases       decrease decrease Down Arrow with Asterisk Down Arrow with Asterisk Down Arrow with Asterisk decrease decrease
11. VMT Pricing primary effect primary effect         decrease decrease decrease decrease decrease decrease decrease
12. Fuel Pricing primary effect primary effect       primary effect decrease decrease decrease decrease decrease decrease decrease
13. Employer-based TDM Programs primary effect primary effect may be a notable effect, but not in all cases       decrease decrease Down Arrow with Asterisk Down Arrow with Asterisk Down Arrow with Asterisk decrease decrease
14. Non-Employer-based TDM primary effect primary effect may be a notable effect, but not in all cases       decrease decrease Down Arrow with Asterisk Down Arrow with Asterisk Down Arrow with Asterisk decrease decrease
15. Land-use Strategies primary effect primary effect     primary effect   decrease decrease Down Arrow with Asterisk Down Arrow with Asterisk Down Arrow with Asterisk decrease decrease
√=primary effect; +=may be a notable effect, but not in all cases; -=may have the opposite effect, in some cases
↓=decrease; ↓*=generally decreases, but possibility of an increase; ↓/↑=varies; ↑=increase; N=no change/not quantified

General Emissions Impacts of Transportation System Management Strategies

Transportation system management (TSM) strategies focus on changing the operation of the transportation system, typically with a primary focus on improving traffic flow and reducing traveler delay. TSM programs can reduce emissions by changing vehicle speeds, reducing rapid vehicle accelerations and decelerations, and reducing vehicle idling. Many of these strategies are under the umbrella of Intelligent Transportation Systems (ITS). In addition, some strategies focus directly on encouraging changes in driving behavior through educational information, incentives, or restrictions on driving speeds, operating patterns, and idling.

The table below provides a summary of the general emissions impacts of selected TSM strategies. Note that strategies that affect vehicle travel speeds will generally show no effect on PM, SOx, and NH3, and might result in either an increase or decrease in CO, NOx, and VOC, depending on the starting vehicle speeds and level of speed change. On the other hand, strategies that reduce vehicle idling will generally reduce all pollutants. Some strategies can either affect both travel speeds or vehicle idling time, or affect both.

Table 2-2: General Emissions Impacts of TSM Strategies

Strategy Category of Primary Effect General Pollutant Effect
Reduce VMT Reduce vehicle trips Shift travel time Reduce idling Change speeds Change vehicle stock PM-2.5 PM-10 CO NOx VOCs SOx NH3
16. Signal Synchronization/Intersection Improvements - -   primary effect primary effect   decrease/no change decrease/no change varies varies generally decreases decrease/no change decrease/no change
17. Incident Management/Traveler Information may be a notable effect, but not in all cases     primary effect primary effect   decrease/no change decrease/no change varies varies generally decreases decrease/no change decrease/no change
18. Speed Control         primary effect   no change no change varies varies varies no change no change
19. Shifting/Separating Freight Movements         primary effect   no change no change varies varies generally decreases no change no change
20. Vehicle Idling Restrictions/Programs       primary effect     decrease decrease decrease decrease decrease decrease decrease
=primary effect; +=may be a notable effect, but not in all cases; -=may have the opposite effect, in some cases
↓=decrease; ↓*=generally decreases, but possibility of an increase; ↓/↑=varies; ↑=increase; N=no change/not quantified

General Emissions Impacts of Vehicle, Fuel and Technology Strategies

Vehicle, fuel, and technology projects and programs are designed to change the emissions rates of vehicles either by changing the fuel being used, the type of vehicle or emissions control technology, or a combination of both. Some programs also focus on eliminating gross polluters, or vehicles whose emissions controls have failed. The methodologies for these strategies generally involve estimating the number of vehicles affected, and then calculating the change in the emissions factors based on changes in vehicle stock or equipment. The specific emissions reductions depend on the type of technology and/or fuel used. The table below provides a summary of the general emissions impacts of selected vehicle, fuel, and technology strategies. It is important to note that the emissions impacts of these strategies vary considerably based on the specific types of technologies and fuels that are used. For instance, some diesel engine retrofits target reducing PM while others focus on NOx.

Table 2-3: General Emissions Impacts of Vehicle, Fuel and Technology Strategies

General Emissions Impacts of Vehicle, Fuel and Technology Strategies
Strategy Category of Primary Effect General Pollutant Effect
Reduce VMT Reduce vehicle trips Shift travel time Reduce idling Change speeds Change vehicle stock PM-2.5 PM-10 CO NOx VOCs SOx NH3
21. Idle Reduction Facilities       primary effect     decrease decrease decrease expected decrease decrease expected decrease expected decrease expected
22. Accelerated Retirement/Replacement of Buses           primary effect decrease decrease decrease decrease decrease decrease decrease
23. Accelerated Retirement/Replacement of Heavy Duty Trucks           primary effect decrease decrease decrease decrease decrease decrease decrease
24. Diesel Engine Retrofits           primary effect (-)/N decrease/no change decrease decrease/no change decrease no change no change
25. Clean Diesel Fuel           primary effect decrease expected decrease/no change varies varies varies decrease/no change no change
26. Inspection & Maintenance Programs           primary effect no change no change decrease decrease decrease no change no change
=primary effect; +=may be a notable effect, but not in all cases; -=may have the opposite effect, in some cases
↓=decrease; ↓/↑=varies; N=no change/not quantified; (-)=decrease expected, but not quantified in EPA guidance

General Emissions Impacts of Non-road Transportation Strategies

Non-road vehicles and equipment include railroads, marine vessels, airport ground support equipment, lawn and garden equipment, construction and agricultural equipment, and other mobile equipment. There are a wide range of technologies and operational strategies available to address these sources. The strategies presented in this report focus on policies and programs that can be implemented at the state and local level. The table below provides a summary of the general emissions impacts of selected non-road transportation strategies. It should be noted that diesel engine retrofits and clean diesel fuels strategies (#24 and 25 above) can also be applied to non-road diesel engines with similar results.

Table 2-4: General Emissions Impacts of Non-Road Transportation Strategies

Strategy Category of Primary Effect General Pollutant Effect
Reduce VMT Reduce vehicle trips Shift travel time Reduce idling Change speeds Change vehicle stock PM-2.5 PM-10 CO NOx VOCs SOx NH3
27. Locomotive Replacement/Repowers           primary effect decrease expected decrease no change decrease decrease no change no change
28. Rail Electrification       may be a notable effect, but not in all cases   primary effect decrease expected decrease decrease decrease decrease decrease expected decrease expected
29. Locomotive Idling Reduction       primary effect     decrease decrease decrease expected decrease decrease expected decrease expected decrease expected
30. Marine Vessel Replacement/Repowers           primary effect decrease expected decrease no change decrease decrease no change no change
31. Marine Vessel Operational Strategies       primary effect     decrease expected decrease decrease expected decrease decrease decrease expected decrease expected
32. Transportation Equipment Replacement/ Repowers           primary effect decrease expected decrease no change decrease decrease no change no change
√=primary effect; +=may be a notable effect, but not in all cases; -=may have the opposite effect, in some cases
↓=decrease; ↓/↑=varies; N=no change/not quantified; (-)=decrease expected, but not quantified in EPA guidance

General Emissions Impacts of Road Dust Strategies

Road dust reduction strategies focus on limiting the amount of particulate matter that is kicked up by the movement of vehicles on roadways. These strategies reduce PM-2.5 and PM-10 from road dust, but typically have no effect on other pollutants, as shown in the table below. In the case of street sweepers, there may be a small increase in emissions of other pollutants if the emissions from the street sweeping equipment are considered.

Table 2-5: General Emissions Impacts of Road Dust Strategies

Strategy Category of Primary Effect General Pollutant Effect
Reduce VMT Reduce vehicle trips Shift travel time Reduce idling Change speeds Change vehicle stock PM-2.5 PM-10 CO NOx VOCs SOx NH3
33. Unpaved Road Dust Mitigation             decrease decrease no change no change no change no change no change
34. Road Paving - -         decrease decrease no change no change no change no change no change
35. Street Sweeping             decrease decrease generally no change generally no change generally no change generally no change generally no change
√=primary effect; +=may be a notable effect, but not in all cases; -=may have the opposite effect, in some cases
↓=decrease; ↓/↑=varies; N=no change/not quantified; N* = generally no change, but possibility of an increase

Interactions between Strategies and Effects

While strategies are classified independently, it is important to recognize that many strategies are not typically implemented in isolation, and consequently should not be evaluated as such. Specifically:


5 In some cases, strategies may have other impacts, such as also altering vehicle speeds, which may increase emissions of one or more pollutants.

6 Emissions reductions will depend on all else remaining equal, i.e., reducing VMT on a facility could cause higher speed thus higher emissions.

7 See EPA's document "Guidance for Quantifying and Using Long Duration Truck Idling Emission Reductions in State Implementation Plans and Transportation Conformity," 2004, http://epa.gov/cleandiesel/documents/420b09037.pdf.

8 For instance, an expansion in transit services could potentially increase PM and NOx emissions if new buses do not take enough cars off the road to offset the emissions associated with increased bus operations. However, in general, a transit service expansion would only be considered as an emissions reduction strategy if there is sufficient ridership expected to yield net emissions benefits.

Updated: 11/02/2011
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