Optimal design of bimodal hierarchical transit systems: Tradeoffs between costs and CO2 emissions

To investigate the environmental implications of a collaborative approach between fixed-route transit and demand-responsive transit, this paper studies a bimodal hierarchical transit system, with a specific focus on the impact of CO2 emissions. In this system, demand-responsive transit serves regions with lower demand, while fixed-route transit extends its service to the central business district. Subsequently, a continuous approximation model is formulated, optimizing critical network design parameters such as stop spacing, line spacing, and headway. This optimization process balances passengers' time costs, agency expenditures, and environmental considerations. A series of comparative analyses are conducted to assess the influence of incorporating emission-related costs into the network design. The inclusion of environmental factors in the design process results in an approximate 8.3% reduction in CO2 emissions. Furthermore, numerical case studies are presented, encompassing a broader spectrum of key design parameters, including the value of time and variations in demand. The findings also demonstrate that the proposed hierarchical transit system exhibits lower CO2 emissions when compared to the paired-line system.