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.