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Dynamic ride-sharing systems enable people to share rides and increase the efficiency of urban transportation by connecting riders and drivers on short notice. Automated systems that establish ride-share matches with minimal input from participants provide the most convenience and the most potential for system-wide performance improvement, such as reduction in total vehicle-miles traveled. Indeed, such systems may be designed to match riders and drivers to maximize system performance improvement. However, system-optimal matches may not provide the maximum benefit to each individual participant. In this paper we consider a notion of stability for ride-share matches and present several mathematical programming methods to establish stable or nearlystable matches, where we note that ride-share matching optimization is performed over time with incomplete information. Our numerical experiments using travel demand data for the metropolitan Atlanta region show that we can significantly increase the stability of ride-share matching solutions at the cost of only a small degradation in system-wide performance.

We develop a robust optimization framework for dynamic empty repositioning problems modeled using time-space networks. In such problems, uncertainty arises primarily from forecasts of future supplies and demands for assets at different time epochs. The proposed approach models such uncertainty using intervals about nominal forecast values and a limit on the system-wide scaled deviation from the nominal forecast values. A robust repositioning plan is defined as one in which the typical flow balance constraints and flow bounds are satisfied for the nominal forecast values, and the plan is recoverable under a limited set of recovery actions. A plan is recoverable when feasibility can be reestablished for any outcome in a defined uncertainty set. We develop necessary and sufficient conditions for flows to be robust under this definition for three types of allowable recovery actions. When recovery actions allow only flow changes on inventory arcs, we show that the resulting problem is polynomially solvable. When recovery actions allow limited reactive repositioning flows, we develop feasibility conditions that are independent of the size of the uncertainty set. A computational study establishes the practical viability of the proposed framework.

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This paper presents a paired-vehicle recourse strategy for the vehicle routing problem with stochastic demands (VRPSD). In the VRPSD, a fleet of homogeneous capacitated vehicles is dispatched from a terminal to serve single-period customer demands which are known in distribution when planning but only revealed with certainty upon vehicle arrival. While most existing research for this problem focuses on recourse strategies where each vehicle operates independently, this paper alternatively considers a strategy in which vehicles may be coordinated in pairs. A tabu search heuristic is developed to find good solutions to VRPSD instances with homogeneous customer demand distributions given this alternative recourse strategy. Finally, a computational study on a set of test problems with a variety of demand distributions reveals that the paired recourse strategy may lead to expected travel cost savings of 3% to 25% on problems with 50 customers or greater.

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