Mixed bus fleet management strategy for minimizing overall and emissions external costs

Diesel buses add substantially to air pollution. To mitigate this problem, more and more clean-energy buses are introduced. Among them, electric bus has been recognized as the cleanest with lower emissions. But the deployment of electric bus is limited by its short travel distance and long charging time. In this paper, based on the approach of remaining life additional benefit-cost (RLABC), we propose an approach called new life additional benefit-cost (NLABC) to solve the mixed bus fleet management (MBFM) problem. An integer program is developed based on the NLABC analysis for maximizing the total net benefit of early replacement, where both the optimal fleet size and composition under budget constraints can be determined. Arguably, the routing problem is a major issue to be tackled due to the range limitations and operating costs of electric buses in the MBFM problem. Hence, we include the routing problem associated with bus services coordination among multiple routes in this formulation. Two routing methods are proposed to solve the recharging problem to study the tradeoff between accuracy and efficiency. Four types of buses, including electric bus, compressed natural gas bus, hybrid-diesel bus, and diesel bus, are considered, while accounting for their different operating costs, external costs of emissions, and purchase costs. To illustrate the approach, we apply the formulation to some transit lines in Hong Kong. The results show that vehicle routing with bus service coordination and mixed fleet optimization are important considerations for managing the bus fleet; both of which can produce considerable benefits.