Inclusion of regional transit emissions in local greenhouse gas inventories

Methods to attribute greenhouse gas emissions from transit vehicles across cities in a multijurisdictional region are explored. Four methods and one submethod are proposed, tested, and evaluated with real-world data from the Bay Area Rapid Transit District, serving the San Francisco Bay Area, California, and the Tri-County Metropolitan Transportation District of Oregon, serving the Portland area. Each methodology is evaluated on the basis of the likely availability of necessary data, ease of calculation, policy implications, and accuracy. Method 1 allocates emissions on the basis of each jurisdiction's total population and employment as a share of population and employment from all of the region's jurisdictions that have transit access. Method 2 allocates emissions on the basis of each jurisdiction's share of vehicle revenue miles traveled within the jurisdiction. Method 3 allocates emissions on the basis of each jurisdiction's share of linked transit trip origins and destinations weighted by trip distances. Method 4 allocates emissions on the basis of each jurisdiction's share of boardings and alightings. The methods have clear differences in the amount and type of data and the complexity of calculations required. These differences can be readily compared with the data and analytical resources available to a region to provide a partial ranking of methods. Questions of fairness, accuracy, and policy incentives are complicated by theoretical challenges in assigning responsibility for transit service as well as by the unique urban and transportation contexts of each region. Each region will need to select the method that is most appropriate for its unique circumstances in order to achieve intraregional consistency.