2.2. Optimal Path Problem in STD Networks. In realworld transportation networks, there exists more than one path connecting the current customer-node with the next unserved one, so taking which path to continue the delivery should be decided based on certain optimality criterions.That is to say, optimal path finding problem between two customer nodes is the fundamental subproblem for VRP and should be addressed first here to cope with the STD nature of travel times. If the uncertainty of link travel times can be captured to determine the optimal path efficiently, approaches may be more easily obtained for STDVRPTW. However, “optimal routes selection” in STD networks is more difficult than in deterministic networks, in part because, for a given departure time, more than one path may exist between an origin and destination, each with a positive probability of having the Discrete Dynamics in Nature and Society 3 least travel time, so the definition of an optimal path can be somewhat indeterminate. Hall [12] first puts forward the stochastic and timedependent optimal path problem (STDOPP). He chooses minimum expected travel time (METT) as the optimality criterion and proposes a branch-and-bound procedure for finding the METT path in STD networks. Miller-Hooks and Mahmassani explore the definition of optimality based on first-order stochastic dominance and definite dominance. After the 1990s, the utility theory in economics has been introduced to solve the STD optimal path problem. Wellman [13] identifies a stochastic consistent condition and presents a revised path-planning algorithm based on utility function. Huang and Gao [14] define a disutility function of travel time to evaluate the STD paths. They design an exact labelcorrecting algorithm to find the optimal path with the minimum expected disutility, but the algorithm had exponential computation complexity. Most existing approaches to this problem in terms of different criterions of optimality rely on the precise probability distributions for link travel times, which is hard to realize in practical application. High computation complexity and inefficient algorithms are also strong restraints when solving large size network problems. In recent years, considering the “worst-case performance” of each path, the robust optimization theory has emerged as a preemptive way to address the uncertainties of link travel times without requiring exact probability distributions. Bertsimas and Sim [15] propose a linear robust optimization approach based on polyhedral uncertainty sets. Sim [16] proposes a new methodology to solve the stochastic optimal path problem, which promises greater computational tractability, both theoretically and practically, than the classical robust framework; however, he does not conduct a further study for solving the optimal path problem in STD networks.
2.3. Summary of Past Literature. Although routing models in STD networks are gaining greater attention in the literature and industry, a general modeling framework and efficient solution algorithm which is applicable to large-scale real-world networks are still lacking. Stochastic and timedependent travel times are more extensively operated on optimal path analysis between two service nodes when executing VRP delivery. However, most of the existing approaches to the STDOPP generally need a precise probability distribution of the uncertain link travel times which is hard to realize in practical application. High computation complexity and inefficient algorithms are also strong restraints when solving large size networks problems. In recent years, robust optimization theory has emerged as a preemptive way to address the uncertainties of link travel times with better computational tractability, meanwhile without requiring the precise probability distribution of link travel times. In this paper, for such delivery routing with rigid arrival time requirements, the worst-case travel times of each candidate path connecting any pair of customer nodes should be considered to guarantee the time window constraint satisfied. Beyond that, the reliability of travel time is also a concern in delivery. So we refer to the robust approach here and apply it to solve the subproblem of STDVRPTW. The path, which minimizes the worst-case travel times over all the candidate paths, is defined as the optimal path connecting any two customer nodes.