In the past few decades, with the increasing development of China’s urbanization process, traffic congestion, especially in large cities, has become increasingly serious, which has attracted scholars’ extensive attention. In order to solve this serious problem, many cities have increased their investment in transportation infrastructure, which makes city roads more intricate and provides more alternative paths for urban distribution. In the traditional pollution emissions problem (PEP) solving process, there is just one route between two customers. The aim of optimization is to determine the customer access order of vehicles in an urban distribution with the goal of minimizing emissions. In these classic pollution routing problems (PRPs), suboptimal alternative paths are eliminated, leaving only one path between two client nodes. Indeed, the quality sequence of the roads will change as time goes by. It is important to consider alternative paths in the new condition. The multi-path vehicle routing problem studied in this paper adds alternative paths into the urban distribution, which makes the vehicle always choose other smoother paths when it encounters traffic jams during distribution, most of which may not be the shortest path. However, the distribution cost of the vehicles is even smaller because delivery vehicles avoid road congestion. Therefore, how to choose the appropriate travel route and then minimize possible travel congestion in the complex road network has become the main challenge logistics companies face.
With the continuous improvement of urban transportation network construction, the optionality of urban roads has been enhanced, which provides a new approach for solving urban congestion. In this condition, an increasing number of scholars have begun to pay more attention to the optionality of paths and have studied road networks showing multi-graph and path flexibility. Garaix et al. [
1] studied the multi-attribute vehicle routing problem and proposed a multi-graph representation of the road network. Setak et al. [
2] solved the time-dependent vehicle path problem in a multi-graph with first-in and first-out (FIFO) properties through a heuristic tabu search (TS) algorithm. Demir et al. [
3] first proposed the pollution emissions problem (PEP), which primarily used the integrated modal emissions model (IMEM) to calculate the vehicle’s fuel consumption. Ehmke et al. [
4] studied the issue of emissions-minimized vehicle routing with time-dependence. Grote et al. [
5] extended PRP to dual-objective PRP, the objective function of which aims to reduce fuel consumption and travel time. In order to minimize travel and fixed costs, Koç et al. [
6] considered the PRP of heterogeneous vehicles and determined the number of each vehicle type and the driving route of these vehicles. Konak and Xiao. [
7] considered emission minimization in the problem of location routing of a heterogeneous fleet and divided the city into different regions, each with a constant (time-independent) speed. When the vehicle travels between different areas with different loads, it can choose different paths between areas to minimize emissions. Barth and Boriboonsomsin [
8] established a time-dependent network model that relies on FIFO attributes. Ichoua et al. [
9] proposed a model with a step function for driving speed and a piecewise linear function for vehicle travel time. This method has been widely used in other studies. Kim et al. [
10] used a heuristic algorithm to solve the time-dependent vehicle routing problem (TDVRP) in dynamic vehicle routing problems and reported that using the time-dependent shortest path in TDVRP can significantly reduce vehicle travel time. Bektas and Laporte [
11] analyzed the pollution routing problem based on emission and energy consumption models; moreover, the effects of time windows, speed, distance and other factors on vehicle emissions were considered. Repoussis et al. [
12] studied the open vehicle routing problem with time windows. Wang et al. [
13] considered the impact of ramp factors on emissions in the vehicle routing problem (VRP) and proposed a two-objective strategy for energy consumption minimizing low-carbon Vehicle routing problems (ECM-LCVRP) in different road gradient environments. Based on the classical TDVRP, Liu and Zhang. [
14] constructed a model of the urban distribution problem that comprehensively considered energy conservation, low carbon and cost saving and further minimized economic cost, including the above three factors; the goal was to plan the vehicle routing problem. On the basis of studying the vehicle fuel consumption model, time-window penalty function and speed optimization strategy, Ge et al. [
15] proposed a variable-speed vehicle routing optimization model with a time window so as to solve the problem of the difficulty of a vehicle traveling with constant speed to meet the time-to-service requirement of its customers. A low-carbon pickup and delivery vehicle routing problem was proposed by Qin et al. [
16], the adaptive genetic hill-climbing algorithm was designed to solve the optimization model, which considers the carbon tax policy. Bravo et al. [
17] analyzed the pickup and delivery vehicle pollution routing problem with multi-objectives, and the total traveling time, the emission of greenhouse gases and the number of customers were considered in the model. An evolutionary algorithm was designed to solve this problem. The multi-objective regional low-carbon location routing problem was proposed by Leng, L.L. [
18]. The total cost, time and service duration were considered in the model, three multi-objective evolutionary algorithms were designed based on the complexity of the proposed problem. Shen, L., et al. [
19] described an open vehicle routing problem with time windows, and the low-carbon open vehicle routing problem with time-windows model was established, and the goal was minimum total costs. A two-phase algorithm was designed to handle the model. Niu, Y.Y, et al. [
20] analyzed the green open vehicle routing problem with time windows. The comprehensive modal emission model (CMEM) was established, and a hybrid tabu search algorithm with several neighborhood search strategies was designed to handle this problem.
Based on the analysis above, it can be concluded that the former research on vehicle routing problems with time windows (VRPTW) and PEP focused on the factors of traffic congestion, vehicle composition and vehicle load, etc., rather than multi-path vehicle routing problems [
21,
22,
23]. Therefore, a multi-path mixed-integer mathematical programming model with a time window was established, which aimed to optimize fuel cost, driver cost, vehicle depreciation and time-window penalty cost and determine the exact solution using CPLEX in Java. Finally, the applicability and feasibility of the model were verified with the example of Jingdong’s (JD) logistics in the main urban area of Chongqing, and sensitivity analyses were performed on this model.