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35 congestion and compact development may be included in age trip length for transit trips and for displaced vehicle trips an analysis of strategies. These components have more will be the same. The impact of displaced vehicle trips on often been included in analyses of the impacts of transit GHG emissions is then estimated using figures for aver- systems than in analyses of transit strategies. age light-duty fuel efficiency from the Energy Information Administration or EPA, and standard factors of GHG emis- The following sections describe proposed and commonly sions per gallon of fuel. used methods for estimating each of the four components of analysis. These techniques generally can be applied to indi- Some recent studies have used a simplified approach to vidual transit projects or strategies, to entire transit agen- calculate displaced travel. Assuming that every trip made cies, or to an aggregate of all transit service in a state or the on transit would be taken by car if transit were not available, United States. The basic analytical principles are the same at these studies have applied national ratios of average vehicle each level, although the specific techniques for data gather- occupancy to calculate the number of car trips displaced. ing and forecasting vary. This approach ignores the possibilities of walking, biking, or not taking a trip as alternatives to transit (16,21). TRAVEL MODE SHIFT Analyses of the total mode shift provided by a transit agency or agencies typically can use empirical data on PMT To estimate the impact of mode shift to transit, transit agen- as reported in the National Transit Database (NTD). Analy- cies must determine how many private vehicle trips are dis- ses of the benefits of specific lines and services require more placed by trips on buses and trains. Some trips on transit detailed data. For large projects, displacement of VMT is remove private vehicles from the road. Other trips made on often analyzed as part of ridership projections or environ- transit would have been made by carpool, walking, biking, mental analysis. For smaller projects, sketch planning tech- or not made at all, if transit were not available. niques may be more appropriate. There are three general approaches to estimating the The Region of Waterloo, which provides transit services mode shift effect for a given transit agency. First, agencies in Waterloo, Ontario (in the greater Toronto region), recently can use regional travel demand models that predict trip pat- estimated the impact of a new bus line on GHG emissions. terns based on transportation networks, land uses, and other The Region of Waterloo used a survey of riders and pas- factors. MPOs typically maintain the travel demand model senger counts to estimate the mode shift effect of the new for a region. Using a travel demand model to calculate mode service. The bus line, termed the iXpress, is a BRT system shift is relatively labor intensive. In addition, many urban that replaced a conventional bus system beginning in Sep- areas' travel demand models do not include a robust meth- tember 2005. The iXpress service was implemented in con- odology for calculating transit trips and are therefore inad- junction with transit signal priority measures, a web-based equate for this type of analysis. trip planner, an automatic passenger counting (APC) sys- tem, an AVLC system, community-based marketing initia- A second approach uses evidence from "natural experi- tives, and inter-modal integration measures. The APC and ments." For example, where transit service has been tempo- AVLC systems will be used specifically to monitor rider- rarily eliminated by strikes or power outages, the empirical ship and to optimize routes and schedules in the future. The impacts on VMT can inform an estimate of travel mode shift. iXpress route extends 35 km (22 mi) and serves 13 stations. The iXpress' better quality service, faster travel times, and A third approach applies a mode shift factor to data on improved connections for pedestrians and bicyclists all con- the transit agency's passenger mileage. APTA's methodol- tributed to increasing ridership on the route. ogy recommends this approach. A mode shift factor is a ratio of transit passenger trips to displaced private auto trips. For The Region of Waterloo, in partnership with the Univer- example, a mode shift factor of 0.5 means that for every 100 sity of Waterloo, estimated the impact of the iXpress service trips made on transit, 50 vehicle trips are avoided. Locally on mode shift by recording the daily passenger boardings on appropriate mode shift factors can be estimated using out- the service and surveying passengers to determine how they puts from a regional travel demand model or from rider made their trips before iXpress became available. Results surveys. In the absence of these types of information, agen- of the survey are presented in Figure 16. Assuming that all cies can use average mode shift factors available in APTA's auto trips are made by SOVs, we can estimate from these methodology. Mode shift factors are provided for various data a mode shift factor of 0.136 for the iXpress. With the sizes of transit agencies. benefit of the detailed individual survey responses, the tran- sit agency conducted a slightly more complex analysis than Applying the mode shift factor to the total number of pas- recommended by APTA's methodology. The analysis used senger trips, agencies can calculate the number of vehicle individual trip lengths by nontransit mode to calculate dis- trips they displace. Typically, analyses assume that the aver- placed emissions, rather than using an average length for all