A Two-Phase Heuristic Algorithm for the Common Frequency Routing Problem with Vehicle Type Choice in the Milk Run

High frequency and small lot size are characteristics of milk runs and are often used to implement the just-in-time (JIT) strategy in logistical systems. The common frequency problem, which simultaneously involves planning of the route and frequency, has been extensively researched in milk run systems. In addition, vehicle type choice in the milk run system also has a significant influence on the operating cost. Therefore, in this paper, we simultaneously consider vehicle routing planning, frequency planning, and vehicle type choice in order to optimize the sum of the cost of transportation, inventory, and dispatch. To this end, we develop a mathematical model to describe the common frequency problem with vehicle type choice. Since the problem is NP hard, we develop a two-phase heuristic algorithm to solve the model. More specifically, an initial satisfactory solution is first generated through a greedy heuristic algorithm to maximize the ratio of the superior arc frequency to the inferior arc frequency. Following this, a tabu search (TS) with limited search scope is used to improve the initial satisfactory solution. Numerical examples with different sizes establish the efficacy of our model and our proposed algorithm

Assessing the role of shared mobility services in reducing travel-related greenhouse gases (GHGs) emissions : focusing on America’s young adults

This study analyzes the relation between shared mobility services and greenhouse gases (GHGs) emissions by using a nationally representative sample of US young adults. We conduct a comprehensive analysis based on the data collected in the 2017 National Household Travel Survey (NHTS). These trip-level emissions are calculated following MOtor Vehicle Emission Simulators (MOVES) and Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model (GREET) workflows. We find that the weekday sample has a significantly higher value in daily GHGs emissions than the weekend sample. Controlling for other factors, shared micromobility services usage is found to have a significant impact on daily GHGs emissions for both weekday and weekend travel. Our analyses further indicate that carsharing complements public transit, and its users are more likely to reside in areas with better public transit supply. We find that the use of transportation network companies (TNCs) has a positive relationship with young adults' GHGs emissions on weekdays only. The study results and implications may be useful for planners and professionals interested in tracking the impacts of new mobility services on transportation and the relevant environmental outcomes

Comparing Unit Trains versus Manifest Trains for the Risk of Rail Transport of Hazardous Materials -- Part I: Risk Analysis Methodology

Transporting hazardous materials (hazmats) using tank cars has more significant economic benefits than other transportation modes. Although railway transportation is roughly four times more fuel-efficient than roadway transportation, a train derailment has greater potential to cause more disastrous consequences than a truck incident. Train types, such as unit train or manifest train (also called mixed train), can influence transport risks in several ways. For example, unit trains only experience risks on mainlines and when arriving at or departing from terminals, while manifest trains experience additional switching risks in yards. Based on prior studies and various data sources covering the years 1996-2018, this paper constructs event chains for line-haul risks on mainlines (for both unit trains and manifest trains), arrival/departure risks in terminals (for unit trains) and yards (for manifest trains), and yard switching risks for manifest trains using various probabilistic models, and finally determines expected casualties as the consequences of a potential train derailment and release incident. This is the first analysis to quantify the total risks a train may encounter throughout the shipment process, either on mainlines or in yards/terminals, distinguishing train types. It provides a methodology applicable to any train to calculate the expected risks (quantified as expected casualties in this paper) from an origin to a destination