Public transit systems with high occupancy can save greenhouse gas (GHG) emissions. However, current transit systems had not been designed to reduce environmental impacts. This motivates the study of the benefits of optimal design and operational approaches to reducing the environmental impacts of transit systems. Transit agencies could resort to level-of-service (LOS) changes, for example, reductions in vehicle kilometers traveled. In previous work, we explored the unintended consequences of lowering transit LOS on emissions from a single-technology transit systems. Herein we extend the analysis to account for a more realistic case: a transit system with hierarchical structure (trunk and the feeder lines) providing service subject to demand elasticity. By considering the interactions between the trunk and the feeder systems, we provide a quantitative basis for designing and operating integrated urban transit systems that can reduce GHG emissions and societal costs. We find that large cities may achieve societal cost as well as emission savings by employing a hierarchical structure, while the non-hierarchical structure may be better for small cities. Highly elastic transit demand may cancel emission reduction potentials, yet for mass transit modes these potentials are still significant. Transit networks with buses, bus rapid transit or light rail as trunk transit modes should be designed and operated near the cost-optimal point when the demand is highly elastic, while this is not required for metro.
Public transit is often touted as a 'green' transportation option and a way for users to reduce their environmental footprint by avoiding automobile emissions, but that may not be the case when systems run well below passenger capacity. In previous work, we explored an approach to optimizing the design and operations of transit systems for both costs and emissions, using continuum approximation models and assuming fixed demand. In this letter, we expand upon our previous work to explore how the level of service for users impacts emissions. We incorporate travel time elasticities into the optimization to account for demand shifts from transit to cars, resulting from increases in transit travel time. We find that emissions reductions are moderated, but not eliminated, for relatively inelastic users. We consider two scenarios: the first is where only the agency faces an emissions budget; the second is where the entire city faces an emissions budget. In the latter scenario, the emissions reductions resulting from reductions in transit level of service are mitigated as users switch to automobile.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.