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34 CHAPTER five ESTIMATING GREENHOUSE GAS SAVINGS FROM TRANSIT Transit agencies can estimate both the impacts of entire tran- CO2 is the most commonly analyzed gas, accounting for sit systems on GHG emissions and the marginal impacts of 95% of U.S. transportation GHG emissions (5). Emissions specific strategies on GHG emissions. Impacts of systems of CO2 are typically the easiest to calculate. CH4 and N2O generally can be quantified using existing data and analysis are also commonly included in calculations. The remaining techniques. Estimating the impact of specific transit strate- gases are less commonly included, although estimates of gies is more complex in some cases. This chapter provides an these are required by some registries. overview of analysis frameworks and some agencies' expe- riences with quantification, as well as references to more TABLE 11 detailed calculation methodologies. TYPICAL SOURCES OF EMISSIONS Global Several recent studies have provided methodologies for Typical Sources for Warming calculating the GHG impacts of transit service. The most Gas Transit Agencies Potential (GWP) robust of these is a methodology developed by APTA's Cli- Gasoline and diesel mate Change Standards Working Group and funded by FTA. combustion The methodology, Recommended Practice for Quantifying Carbon dioxide Combustion at stationary 1 Greenhouse Gas Emissions from Transit, was released in (CO2) sources; e.g., maintenance 2009. This chapter draws heavily on that document. More yards detail on many of the calculation methodologies described Electricity purchases here is available within APTA's methodology (50 ). Gasoline and diesel combustion Methane (CH4) 21 Quantifying the impacts of transit systems and of transit Fugitive emissions of strategies on GHG emissions is a relatively new effort. For natural gas years, state DOTs, MPOs, and transit agencies have esti- Nitrous oxide Gasoline and diesel 310 mated the impact of transit strategies on criteria pollutants, (N2O) combustion as required by the Clean Air Act; however, there is no regu- Hydrofluorocar- Leakage of refrigerants 1211,700 latory requirement to estimate the impacts of transit on GHG bons (HFCs) emissions. Still, transit agencies may find it useful to quan- Perfluorocarbons tify impacts on GHG emissions for the following purposes: Leakage of refrigerants 6,5009,200 (PFCs) Sulfur hexafluo- Leakage from electrical Reporting to the Climate Registry and other agencies 23,900 ride (SF6) equipment Preparing for possible state and federal reporting Source: Recommended Practice for Quantifying Greenhouse requirements (e.g., Washington State is currently Gas Emissions from Transit: Draft, APTA Climate Change developing a rule that will require reporting by any Standards Working Group, Mar. 2008, p. 16 (50). agency that operates an on-road vehicle fleet that emits at least 2,500 metric tons of GHG annually). Analyses of transit's impact on GHG emissions can Supporting internal efforts to reduce emissions include any of the four components discussed in chapter Communicating the benefits of transit to the public and three. Transit displaces emissions through travel mode to legislators shift, compact development, and reduced congestion. Ensuring eligibility for new funding sources Transit produces emissions from vehicles, facilities, and Preparing for the implementation of new state GHG regula- construction and maintenance (see Figure 5 for a diagram tions, such as California's SB 375, which requires MPOs to of components). Analyses of individual transit strategies plan for reduced GHG emissions from light-duty vehicles incorporate emissions produced by transit vehicles and Preparing for possible new federal regulations. emissions displaced by transit to measure the net impact of strategies. Emissions displaced include, at a minimum, the Analyses can include any of the six GHGs listed in Table impact of travel mode shift. To provide a more complete 11. Within analyses of GHG emissions from transportation, account of displaced emissions, the benefits of reduced