A lexicographic bi-objective approach to fleet sizing and routing for service vehicles in a real-world passenger transport system

The Société de transport de Montréal (STM), a public transport agency in Montreal (Quebec, Canada), has committed to electrifying its entire vehicle fleet as part of its broader sustainability initiatives. This research addresses a critical challenge that arises from this commitment: optimizing the deployment and dispatching of electric service vehicles assigned to operations supervisors to ensure minimal disruption to the bus network. The problem is formulated as a mixed-integer linear program that minimizes the number of deployed vehicles and total response time using a lexicographic approach. The effectiveness of our approach is evaluated through computational experiments using the commercial CPLEX optimization solver. This research provides the STM with a valuable strategic and operational decision-support tool, aimed at optimizing the size and deployment of its new electric vehicle fleet, therefore supporting its sustainability objectives.