1. Introduction
Emissions from transportation systems are considered to be an important component of environmental pollutants. With the expansion of transportation systems and road facilities, emissions from road traffic have been increasing over the years, and the environmental impacts of traffic emissions are receiving increasing attention from researchers [
1]. The sustainable development of road transportation in terms of low emissions of carbon dioxide (CO
2) and criteria pollutants is becoming a requirement and challenge for the transportation of planning designers. Although many measures for alleviating the impact of traffic on the environment (e.g., the adoption of alternative energy vehicles) have been developed, traffic planners should propose solutions with a broader perspective, such as through road network design [
2]. To this end, it is critical to quantify the impact of road traffic on the environment when designing transportation networks, not only with the aim of minimizing the system travel time (or associated cost), but also seeking to minimize the negative impacts of traffic on the environment [
3,
4]. Some researchers have investigated transportation network design problems (NDPs) in the context of environmental considerations, such as minimizing emission and traffic noise from different perspectives, but to our knowledge not in a context involving a heterogeneous traffic stream consisting of connected and autonomous vehicles (CAVs) and human-driven vehicles (HVs) [
5,
6].
CAVs have been undergoing rapid development [
7,
8,
9]. It has been widely accepted that CAV use will be a part of future transportation systems. On the other hand, the transformation from HVs to CAVs will not be accomplished overnight. Instead, a gradual development process is expected. As such, heterogeneous traffic streams consisting of CAVs and HVs will likely persist for a long time. Compared to HVs, the use of CAVs leads to many benefits to the transportation system, including improvements in road capacity [
10,
11] and traffic operation [
12,
13], as well as reduced emissions [
9,
14] and enhanced road safety [
15,
16]. In particular, compared to HVs, CAVs are estimated to reduce emissions by 20–50%, thus offering huge potential for the promotion of sustainable transportation [
17,
18]. Road traffic driving conditions affect the benefits of CAVs, because the intelligent driving of CAVs is dependent on communications equipment and roadside facilities. Some scholars have proposed implementing dedicated CAV lanes in existing road networks to improve system efficiency by offering a better driving environment for CAVs, which would also reduce system travel time [
19]. However, most existing studies only focus on the optimal travel time of the system. In other words, a multi-objective optimization approach that takes into account both system travel time and emissions is necessary to understand the best ways to implement CAV lanes. To the best of our knowledge, no such research exists in the literature.
To fill this gap in the study of sustainable dedicated CAV lane design, in this paper we consider a bi-level multi-objective mathematical programming approach that considers objectives related to both system efficiency and sustainability. The upper level of the proposed model seeks the optimal CAV lane implementation schemes taking into account system travel time cost and emission cost, while also minimizing CAV lane construction cost. The lower-level captures multi-class network equilibrium consisting of two classes of vehicles, i.e., CAVs and HVs. To solve the bi-level multi-objective optimization model, an integrated solution framework combining the non-dominated sorting genetic algorithm II(NSGA-II) and diagonalized Frank–Wolfe (DFW) algorithm was developed. The NSGA-II algorithm is used to find the optimal CAV lane implementation schemes, while the DFW algorithm is used to solve multi-class network equilibrium at the lower-level. In this work, the multi-class network equilibrium addresses the heterogeneous traffic stream consisting of CAV and HV, in which we assume that there needs to be no crisis effect between CAVs and HVs, because CAVs are regarded as HVs when driving in regular lanes. Additionally, we assume driving behavior (machine-to-human) to be ideal, without the consideration of driving crises. On the other hand, any potential crisis between CAVs is negligible when the CAVs are driving in dedicated CAV lanes, because CAVs are expected to experience better driving conditions as a result of their communication equipment and the roadside facilities provided by the CAV lanes.
Broadly speaking, sustainable dedicated CAV lane design belongs to the class of transportation network design problems. Such problems are inherently challenging to solve because: (1) the equilibrium constraints make the domain of feasible solutions nonconvex; (2) constructing the Pareto frontier for different objectives is a non-deterministic polynomial (NP) hard problem. In addition, solving multi-class (CAVs and HVs) network equilibrium needs dealing with the asymmetric impact on link cost between the two vehicle classes. Researchers convert multi-class network equilibrium to mixed-integer linear programs by non-linear complementary conditions in previous studies, which are then solved by solvers like CPLEX Optimizer [
20,
21]. However, solving such mixed-integer linear programs may still take a long time, because the equilibrium constraints make the domain of feasible solutions nonconvex. In this paper, we instead propose a diagonalized Frank–Wolfe algorithm, which is simple to implement and demonstrates good computational performance. On the other hand, we adopted the NSGA-II algorithm to handle the multi-objective and NP hard nature of the problem, which presents a better way to handle multiple objectives with different count units than traditional means such as the weighting method. Another strength of NSGA-II is that it can provide a large number of Pareto solutions, which will be desired for decision-making.
Moreover, some findings are obtained based on numerical examples. First, implementing dedicated CAV lanes could reduce the total time cost, total emission cost, and total energy consumption by about 12.03%, 10.42%, and 9.4% in the Nguyen–Dupuis network. Second, implementing dedicated CAV lanes could further reduce the total travel cost and emission cost with CAV market penetration evolution. Third, expanding the capacity of dedicated CAV lanes will benefit the system’s performance. In detail, compared to two times of capacity, the three times reduce the total travel cost and total emission cost by 5.5% and 4.6%, respectively, when the CAV market penetration is 50%. Finally, the relation between minimizing the total travel time cost and total emission cost is not always consistent, which is affected by vehicle travel speeds. In a nutshell, the contribution of this work lies in the following aspects.
(1) The proposed sustainable dedicated CAV lanes design problem is a novel research issue since it considers economic and vehicle emissions to capture the optimal CAV lanes implement scheme.
(2) A multi-objective bi-level programming model with multi-class network equilibrium constraints was developed to draw sustainable dedicated CAV lanes design problems.
(3) An integrated solution framework combining the meta-heuristic algorithm and diagonalized Frank–Wolfe (DFW) algorithm was designed to solve the dedicated CAV lanes design problems. Numerical experiments are finally conducted to test the proposed solution algorithm on different sizes of networks.
The rest of this study is organized as follows.
Section 2 reviews the literature on CAVs and transportation network optimization problems. The problem formulations and the multi-objective bi-level model are presented in
Section 3 and
Section 4, respectively.
Section 5 reports the detailed solution algorithm.
Section 6 shows the numerical experiment. Finally,
Section 7 summarizes the conclusions and suggestions for future research.
5. Solution Algorithm
In this section, two algorithms are introduced to solve the dedicated CAV lanes optimal implement problem. The upper-level model was to determine the optimal implementation scheme by minimizing the objective value, which is difficult to handle because it belongs to an NP (non-deterministic polynomial) hard problem. Heuristic algorithms are generally regarded as effective methods to solve NP hard problems. Hence, in this paper, the non-dominated sorting genetic algorithm II (NSGA-II) [
44] was adopted to solve the upper-level model, which has multiple objectives. The low-level problem is the multi-class network equilibrium, which can be solved by the diagonalized Frank–Wolfe (DFW) algorithm [
45].
Figure 2 draws the overall framework of the solution approach.
NSGA-II, introduced by Deb et al., is an effective algorithm to deal with multi-objective optimization problems, which is capable of efficiently constructing the Pareto front by three operators: (1) non-dominant solution sorting operator; (2) individually crowded distance operator; and (3) elite strategy selection operator.
Figure 3 shows the detailed flowchart of NSGA-II.
Due to the asymmetric impact on link cost between travel modes, it is difficult to solve the heterogeneous traffic streams equilibrium problems directly. The diagonalization algorithm as an efficient method usually is adapted to handle the heterogeneous traffic streams equilibrium problems. The diagonalization algorithm was first proposed by Abdulaal and LeBlanc [
46] to solve the traffic assignment problem, and then Sheffi [
47] implemented it for the traffic equilibrium problem with asymmetric link interactions.
Note that the diagonalization algorithm is usually combined with a single-class traffic assignment algorithm to solve mixed traffic assignment problems. Specifically, the Frank–Wolfe algorithm is adopted in this work, i.e., a diagonalized Frank–Wolfe (DFW) algorithm is used to deal with the multi-class network equilibrium problems. The detailed steps of DFW are shown in Algorithm 1.
Algorithm 1 Diagonalized Frank–Wolfe algorithm |
Input | Network , CAV implement schemes, Algorithm parameters |
Step 0 | Initialization. Given the CAV implement scheme, perform an all-or-nothing assignment for different users in the network. |
Step 1 | Diagonalization. Find a feasible link-flow vector . Set n:= 0 and update the link travel time. Divide the two types of traffic flows into sub-problems. |
Step 2 | Solve subproblem. Solve the equilibrium problem separately for the users. During each user equilibrium process, first fix the links flows of HVs (or respectively CAVs) as the background flow, and then get ready to solve the UE of only CAVs (or respectively HVs) by traffic assignment algorithm (Frank–Wolfe). This step yields a link-flow vector . |
Step 3 | Convergence test. If , stop; If not n:= n + 1, go to Step 1. |
The sub-problem mentioned in Algorithm 1 refers to the UE problem. In other words, the solution of the sub-problem is equal to the solution of user equilibrium. The UE solution can be obtained by solving following Beckmann’s transformation:
7. Conclusions
We developed a novel dedicated CAV lanes optimal design problem with practical significance and environmental considerations in this work. The vehicle emission as an optimization objective was introduced. We established a multi-objective bi-level programming model with multi-class network equilibrium constraints for the proposed problem. The upper level involves implementing dedicated CAV lanes under multi-objective value, and the lower-level determines the equilibrium flow. The equilibrium constraints are formulated by a set of nonlinear complementarity constraints that characterize the vehicle choice and route choice behavior. To solve the proposed dedicated CAV lane optimal design problem, we propose an integrated solution framework that integrates the meta-heuristic algorithm, diagonalized algorithm, and Frank–Wolfe algorithm. Finally, two numerical experiments demonstrate the effectiveness of the proposed method. It was found that the proposed dedicated CAV lanes implementation program contributes to reduce the network travel time and emission.
We implemented the model and solution algorithm in two classic network cases, which illustrates the efficiency and effectiveness. Several findings are obtained based on numerical examples. First, implementing dedicated CAV lanes could reduce the system total time cost, total emission cost, and total energy consumption by about 2.03%, 10.42%, and 9.4% in the Nguyen–Dupuis network. Second, implementing the dedicated CAV lanes could further reduce the total travel cost and emission cost with CAV market penetration evolution. Third, as a dedicated CAV lane is expected to have greater capacity than a regular lane, the system performance will also benefit from the overall capacity increase by implementing dedicated CAV lanes. For example, when CAV market penetration is 50%, a three-fold capacity increase by converting a regular lane to a dedicated CAV lane would reduce the total travel cost and total emission cost by 5.5% and 4.6%, respectively, compared to a two-fold capacity increase. Finally, we found that minimizing the total travel time cost and minimizing the total emission cost are not necessarily always aligned, and can be affected by vehicle travel speeds. Overall, this research provides a modeling and analyzing framework that can be used to inform planning and decision-making for dedicated CAV lane implementation in future urban transportation networks. The revealed Pareto relationship between construction cost and system performance offers insights into the potential returns of investment under the limited budget. The emission and energy use analysis presented in this paper further lends support to assessing the environmental and energy impact of CAV use and improve policies related to CAV infrastructure development.
There are some future research issues worth investigating. First, we can focus on the CAV demand evolution study. For example, we can establish an applicable CAV forecasting model to capture the CAV demand change. Second, it is meaningful to consider lane capacity change with the CAV market penetration to improve model accuracy. The relationship between the lane capacity and CAV market penetration needs more discussion. Finally, it is useful to study another effective method to solve the model. For instance, the decomposition algorithms and linearization-based solution approach.