Tactical design of last mile logistical systems

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Stroh, Alexander M.
Erera, Alan L.
Toriello, Alejandro
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Tactical Design of Last Mile Logistical Systems Alexander M. Stroh 161 Pages Directed by Dr. Alan Erera and Dr. Alejandro Toriello This dissertation consists of three distinct logistical topics, unified by a focus on the intelligent design of last mile logistical systems at a tactical level. The three design problems all arise within package delivery supply chains, though the mathematical models and solution techniques developed in these studies can be applied to other logistics systems. We propose models that do not attempt to capture granular minute by minute operational decision making, but rather, system behavior on average so that we may approximate the impact of various design choices. In Chapter 2, we study tactical models for the design of same-day delivery (SDD) systems. While previous literature includes operational models to study SDD, they tend to be detailed, complex, and computationally difficult to solve. Thus, such models may not provide any insight into tactical SDD design variables and their impact on the average performance of the system. We propose a simplified vehicle dispatching model that captures the average behavior of an SDD system from a single depot location by utilizing continuous approximation techniques. We analyze the structure of vehicle dispatching policies given by our model for various families of problem instances and develop techniques to find optimal dispatching policies that require only simple computations. Our models can help answer various tactical design questions including how to select a fleet size, determine an order cutoff time, and combine SDD and overnight order delivery operations. In Chapter 3, we study the tactical optimization of SDD systems under the assumption that service regions are allowed to vary over the course of each day. In most existing studies of last mile logistics problems, service regions are assumed to be static. Service regions which are designed too small or cutoff SDD availability too soon may potentially lose SDD market share, while regions which are designed too large or accept orders too late may result in costly operations or failed deliveries, resulting in a loss of customer goodwill. We use a continuous approximation approach to capture average system behavior and derive optimal dynamic service region areas and tactical vehicle dispatching policies which maximize the expected number of SDD orders served per day. Furthermore, we compare such designs to fixed service region designs or capacitated service region designs. In Chapter 4, we introduce the concept of cycle time considering capacitated vehicle routing problems, which are motivated by the desire to decrease the average time packages spent within a delivery network. Traditional vehicle routing models focus on the resource usage of the system whereas our models instead consider the impact of routing policies on the units being served. We explicitly consider pre-routing waiting times at a depot, total demand-weighted accumulated routing times, vehicle capacity constraints, and designing repeatable delivery routes in our models. We present two set partitioning formulations for such problems and derive efficient solution techniques so that the impact of various design parameters can be assessed.
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