A Hybrid Fixed and Flexible Transportation Service: Description, Viability, Formulation, Optimization and Heuristic

Most transportation systems fall into two broad categories: fixed route systems that are cost efficient but lack of flexibility and demand responsive transit (DRT) systems which are flexible but costly. The Mobility Allowance Shuttle Transit (MAST) service is a new concept in transportation merging the flexibility of DRT systems with the low cost operability of fixed route systems. Vehicles follow a base fixed route composed by a few mandatory checkpoints conveniently located at major connection points; given an appropriate slack time, vehicles may deviate from the fixed path within a proper service area to serve passengers at their desired locations. We look at the system from a design, optimization and operational points of view. We first develop relationships between the longitudinal velocity of the vehicles and the demand by using continuous approximations to aid in the design of the service. Then, we provide its MIP formulation and we develop a set of valid inequalities to strengthen it. Finally, we develop a customized insertion scheduling algorithm, whose control parameters prevent a 'wild' consumption of the slack time and significantly improve the operational performance.