Environmental concerns due to fossil fuel consumption and emissions drive transportation industry to shift towards low-impact and sustainable energy sources. Public transit system, as an integral part of multimodal transportation ecosystem, has been supporting such shift by exploring the adoption of electric vehicles. In recent years, the advancement in Battery Electric Buses (BEBs) and their supporting infrastructure technology made them a viable replacement for diesel and Compressed Natural Gas (CNG) buses. Yet, it remains a challenge on how to optimally deploy the BEB system due to its unique spatio-temporal characteristics.
Utah Transit Authority (UTA), the public transportation provider throughout the Wasatch Front of Utah, has already begun the electrification of its bus fleet starting from 2016. 5 BEBs have been brought to service, among which three were used on route 2 and two served the University of Utah campus. After the successful initial release of BEBs , UTA has been working with the University of Utah to further study the possiblility of full electrification.
While BEB and its supporting infrastructure have been commercialized and gradually adopted, how to optimally deploy the BEB system remains a challenge due to several unique spatio-temporal characteristics associated with the system itself. First, to support long daily operation time and high daily mileage, some BEBs would require both periodic on-route charging at bus terminals and overnight charging at bus garages. A careful planning for the optimal locations of on-route charging stations and overnight in-depot charging stations is necessary to efficiently serve the BEBs while keeping the cost minimal. Second, the space-time trajectories of BEBs should fit into current transit vehicle operation routes and schedules as much as possible, to enable smooth transition from traditional diesel or Compressed Natural Gas (CNG) buses to BEBs. The concern for potential interference with current operation routes and schedule would impede the acquisition of BEBs. It thus requires a sophisticated spatio-temporal analytical method to determine how to spatially and temporally integrate BEBs into current public transit system without interference with current operation routes and schedules.
Wei et al. (2018) developed an innovative spatio-temporal analytical framework to assist transit agencies in identifying the optimal deployment for the BEB system. Specifically, a spatio-temporal optimization model is developed to minimize the cost of replacing a certain number of diesel or CNG buses (part of the fleet) with BEBs, while in compliance with existing bus operation routes and schedules. The proposed model can be used to determine the optimal spatiotemporal allocation of the BEBs, as well as the associated on-route charging stations and in-depot charging stations. The network data is obtained from UTA in year 2016.
In addition, Yirong et al (2020) futher enrich the strategical deployment framework of BEB by incorporating a second objective, environmental equity. The research develops a bi-objective spatio-temporal optimization model for the strategic deployment of BEB. The first objective is to minimize the cost of purchasing BEB and installing both on-route and in-depot charging stations while maintaining current bus schedules. The other objective is to maximize environmental equity by incorporating the disadvantaged population in the decision-making process. One main reason is that research on social vulnerability found that low socioeconomic status (SES) groups often experience a higher concentration of air pollutants, due to the low value of lands and the closeness to income-earning opportunities. The formulation is presented on the right-hand side.
The bi-objective optimization framework is structured as following: