Comparison of fuel-cycle emissions per passenger mile from multiple bus and rail technologies

This paper examines the fuel-cycle passenger-mile emissions from multiple bus and rail technologies using a travel-activity-based bottom-up approach. There is abundant literature on transit's emissions savings, but most prior studies rely on the top-down fuel consumption approach. While the top down approach paints a broad picture of transit emissions on a national or regional level, it cannot reflect changes in emissions as a function of operational characteristics such as passenger loading. It is also difficult to estimate passenger-mile emissions of new vehicle/fuel technologies for which fuel economy data are hard to obtain using the top-down approach. This paper develops a unified load-based methodology framework that compares fuel-cycle passenger-mile emissions across transit technologies including alternative fuels and advanced technologies. The methodology reflects the intricate trade-offs between the increased total emissions and decreased passenger-mile emissions as passenger load increases under specific local meteorological and route settings. Using the load-based methodology, the paper demonstrates the varying levels of emissions savings of transit fuels and technologies giving different passenger loadings scenarios. The methodology presented in this paper serves as the foundation of a transit emissions calculator currently under development. The calculator will prove instrumental for local transit agencies to compare fuel/technology alternatives in terms of emissions savings.