2. Literature Review
This literature review is split into two parts: firstly, the literature related to the vehicle routing problems in terms of time dependency and randomness of link travel times is overviewed and secondly, the research on optimal path problem in STD networks is discussed. 2.1. Vehicle Routing Problem. Vehicle assignment and routing problems have been studied for several decades. Most traditional methodologies for this class of problems have been proposed based on adaptations of static algorithms and developed under static travel time, but they less consider dynamic traffic flow conditions [3]. As an extension of VRP, in order to consider possible variations of travel times in the network, Dynamic Vehicle Routing Problem is proposed. Malandraki and Daskin [4] put forward a strict mathematical model for TDVRP. They treat the travel time between two customers as a function of distance and the time of the day and design a nearest-neighbor heuristic algorithms and a cut plane heuristic algorithm. Ichoua et al. [5] introduce a First in First out (FIFO) principle into TDVRP and use time dependent function of speed to represent dynamic network, which avoids the deficiency of waiting in customer node in Malandraki and Daskin’s model [4]. Donati et al. [6] extend this line of research by indicating the importance of optimizing the starting time and designing a more efficient heuristic algorithm in a time-dependent environment Recently, many research works adopt the timedependent speed assumption proposed in [5]. Using queuing models for time-dependent speeds, analytical expressions for the expected travel times as well as for the variance of the travel times are derived in [7]. A continuous function model of time-dependent link speeds is proposed in [8]. More realistic time-dependent information is obtained from archived historical travel data, as seen in [9]. A general modeling framework with finer time-dependent traffic information and efficient solution algorithm are proposed in [1] and are applicable to large-scale real world. In this paper, we consider the VRP with dynamic and stochastic travel times (STDVRP), while there is limited research related to this topic. Lecluyse et al. [10] address the STDVRP by capturing the uncertainty in an analytical way using queuing theory and introduced the variability in traffic flows into the model, which allows for an evaluation of the routes based on the uncertainty involved. Nahum and Hadas [11] combine two important variants to form and define the STDVRP. Two algorithms for solving the stochastic time-dependent VRP are compared. As Ichoua et al. [5] indicate, stochastic and time-dependent travel times are more extensively operated on optimal path analysis between two service nodes when executing VRP delivery (see details in Section 2.2). Although mean and variance contain the most important information about path travel time, finding the single route with expected shortest travel time is not appropriate for routing when planners are not risk neutral. So in order to take account of the STD travel times in STDVRPTW, the optimal path problem in STD networks should be addressed as a subproblem first.