Back to Basics: Demand, Supply, and Emissions Analysis for Urban Mobility Interventions

Climate change concerns have led policymakers and planners to focus on reducing the emissions of heat-trapping greenhouse gases (GHG), in particular carbon dioxide (CO2). The transportation sector is a significant source of GHG emissions resulting from burning petroleum derived gasoline or diesel fuels. One approach to lowering emissions is to reduce vehicle use through travel demand management (TDM) strategies and/or the design and operation of transportation systems. For example, bus rapid transit (BRT) or traffic flow improvement measures can be implemented all with the goal of reducing personal auto use and consequently emissions. The main objective of this paper is to provide a conceptual and analytical means for understanding the relationship between GHG emissions and the main types of urban transportation system interventions generally contemplated to reduce these emissions. To do so, the paper will go back to the basics, and provide a framework for conceptualizing the key phenomena and transportation system interrelations that determine emissions, and use that as a basis for characterizing the methodologies needed to evaluate the emissions impacts from transportation measures. Based on analytical examples of purposefully simplified situations, the importance of considering the interdependence between users and system performance in emissions analysis is illustrated. In particular, the results show that the redistribution of users due to traffic flow improvements, such as improved capacity, may affect the emissions in counterintuitive directions due to these interdependencies. Furthermore, improvements that rely on mode shifts, such as BRT, require a significant shift from auto to bus for even modest reductions in emissions. Additionally, from a conceptual standpoint and the analysis results, this study suggests that methods for analyzing air pollutant emissions may not be directly transferable to GHG emissions.