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Focusing on a freeway lane running with mixed semi-autonomous vehicles (semi-AVs) and fully-autonomous vehicles (fully-AVs), this study aims to investigate the heterogeneous platooning effect of the two types of autonomous vehicles (AVs) on the travel efficiency of the mixed traffic. Such impact in the mixed semi-/fully-AV traffic, to the best of our knowledge, has not been well addressed in the literature as the existing mixed traffic studies on platooning effect typically focused on the traffic scenes with only human-driven vehicles (HVs) and fully-AVs. The existence of semi-AVs, which are optionally controlled by human drivers and semi-autonomous driving systems, and their significant difference in platooning behavior from the fully-AVs have received very limited attention. To fill the gap, we first characterize the semi- and fully-AVs' difference in connection ability and willingness to platoon in the vehicle platooning process, and derive the heterogeneous platooning probabilities of semi- and fully-AVs. To formulate the platooned traffic after vehicle platooning process, we proceed to develop a novel Markov chain model to determine the probability distributions of vehicular platoon size and headway type among the mixed semi-/fully-AV traffic analytically. Furthermore, with the probability distributions, we derive the fundamental diagram (FD) and the capacity of the freeway lane for the mixed semi-/fully-AV traffic, and are thus able to investigate impact of platooning on the travel efficiency numerically. The developed models and unveiled insights in this study will help researchers and practitioners better understand the role, potential and prospect of automated transport system in the transitional stage with semi-AVs, which will be useful for guiding the mixed AV traffic management and control in near future. • Heterogeneous platooning behavior of semi- and fully-AVs is modeled. • Closed-form solutions for platoon size and headway type are derived. • A general approach to the FD and capacity is proposed. • Managerial insights into platooning impact of semi-AVs are provided. [ABSTRACT FROM AUTHOR]

Traffic at urban intersections constitute a series of complex conflicting vehicular movements and contribute greatly to the problems of traffic congestion and air pollution. The emerging of connected and automated vehicle (CAV) technologies inspires innovative ideas in traffic management at intersections; it not only enables new control paradigms, but also allows for possibilities to revolutionize the geometric layouts of intersections. This study proposes a novel intersection design called knotted intersection (KI), to resolve the complexity of conflicting relations at intersections in a full CAV environment. This new design is associated with a set of neat control rules for vehicles to realize the smooth traffic operation at these intersections. The design problem of intersection geometry and control strategy is optimized jointly and formulated as a mixed-integer linear program (MILP), and some basic theoretical properties of KI design are derived, including nearly throughput-maximizing property and bounded within-intersection delay of vehicles. Furthermore, the KI design is then extended to a road network composed of multiple intersections to improve the overall efficiency. The optimal design of multiple knotted intersections (MKI) is developed to coordinate the traffic organization methods among intersections and a two-level solution method is proposed to solve the optimization efficiently. Numerical and simulation experiments are conducted at both the intersection level and network level under various cases, and the results validate the superior performance of the proposed methods compared to classical traffic signal control (TSC) strategies. • Knotted intersection design for resolving the complexity of intersection conflicts • Joint design of intersection geometric layout and traffic control strategy • Theoretical properties for nearly-maximum throughput and bounded delay • Optimal design of multiple knotted intersections to enhance network efficiency [ABSTRACT FROM AUTHOR]

Connected and Autonomous Vehicles is a technology that will be disruptive for all layers of traffic control. The Lagrangian, in-the-flow nature of their operation offers untapped new potentials for sensing and actuation, but also presents new fundamental challenges. In order to use these vehicles for traffic state reconstruction and control, we need suitable traffic models, which should be computationally efficient and able to represent complex traffic phenomena. To this end, we propose the Front-tracking Transition System Model, a cell-free modelling approach that can incorporate Lagrangian measurements, and has a structure that yields itself to on-line model learning and control. The model is formulated as a transition system, and based on the front-tracking method for finding entropy solutions to the Lighthill–Whitham–Richards model. We characterize the solution of this model and show that it corresponds to the solution of the underlying PDE traffic model. Algorithms for traffic state reconstruction and model learning are proposed, exploiting the model structure. The model is then used to design a prediction-based control law for stop-and-go wave dissipation using randomly arriving Connected and Autonomous Vehicles. The proposed control framework is able to estimate the traffic state and model, adapt to changes in the traffic dynamics, and achieve a reduction in vehicles' Total Time Spent. • Connected and Automated Vehicles can be used as traffic sensors and actuators. • Front-tracking Transition System Model is based on front tracking numerical methods. • The model captures stop-and-go waves and moving bottlenecks. • Algorithms are proposed for traffic state reconstruction and model learning. • Prediction-based moving bottleneck control is used to dissipate stop-and-go waves. [ABSTRACT FROM AUTHOR]

• A virtual risk computation based on matching degree was introduced. • A universal planning method for pre-merging in mixed traffic and regular car-following has been proposed. • A virtual risk-based longitudinal planning model has been formulated. • Enhanced stability and improvements in safety are characteristics of the proposed planning approach. Ramp-merging zones have perennially served as bottlenecks for both safety and efficiency within road traffic. Addressing the inherent uncertainties and anomalous behaviors potentially exhibited by human-driven vehicles (HDVs) in mixed traffic flow, this study introduced a distributed merging control algorithm based on virtual risk assessment. Initially, we proposed a progressive vehicle projection method based on matching degree, formulated to engender smooth longitudinal accelerations. Building upon this foundation, a virtual risk-based longitudinal planning model named V-RQM was established, furnishing longitudinal accelerations with robust stability and safety to Connected and Autonomous Vehicles (CAVs). Results from four diverse simulation experiments corroborate that V-RQM is aptly versatile, suited for conventional one-dimensional car-following environments as well as pre-merging control in heterogeneous traffic flow. The model not only mitigates oscillatory amplitudes while sustaining high traffic efficiency but also exhibits commendable resilience against uncertainties and anomalies indigenous to mixed traffic flow. This research pioneers a novel model-driven approach to ramp-merging control, setting the stage for the full exploitation of CAV capabilities. [ABSTRACT FROM AUTHOR]

To reduce energy consumption and transportation emissions during car-following behavior on highways, we propose a car-following model for connected automated vehicles (CAVs). This new model considers spacing variation of the surrounding vehicles located immediately upstream and downstream. We optimize the key parameter of forward control weight of the proposed CAV model to stabilize traffic flow and ensure stable conditions. Then simulation experiments are conducted to validate the effectiveness of our proposed CAV model in enhancing traffic flow stability and energy-saving in mixed traffic consisting of both CAVs and regular vehicles (RVs). Results show that traffic flow controlled by the proposed CAV model is less affected by disturbances. The increase of CAV penetration rates can gradually improve stability of the mixed traffic flow. CAVs equipped with our proposed model can effectively reduce energy consumption and transportation emissions, which decrease with an increase of CAV penetration rates under constant speed conditions. When CAV penetration rate reaches 100 %, the average reduction of energy consumption, CO 2 emissions, and NO x emissions at various speeds reach a peak value of 22.15 %, 31.00 %, and 56.41 %, respectively. Speed also has significant influence on energy consumption and emissions, with potential reductions when speed falls within an appropriate range. • Propose a car-following model of connected automated vehicles (CAVs) for energy-saving. • Present an optimization design for our proposed model to stabilize traffic flow with CAVs. • Conduct simulation experiments to validate the effectiveness of the proposed CAV car-following model in energy-saving. [ABSTRACT FROM AUTHOR]

Urban traffic control (UTC) is inherently multiscale in both temporal and spatial domains. With the wide deployment of connected and automated vehicles (CAVs), data is increasingly available to help reveal this multiscale nature of UTC and the inter-dynamics among different scales. This paper applies the multiscale UTC framework we proposed earlier and extends it to UTC on a network of traffic signals with a mixed flow of CAVs and human-driven vehicles (HDVs). We adopt distributed control as the basic scheme for network-wide control and use information sharing to achieve cooperation among different intersections. We use CAV information to estimate HDV state and develop a "safety check" technique to control CAVs in the mixed traffic flow. Together, we propose a network-wide UTC framework with mixed traffic flow. To address the computation issue of the model-based multiscale method, we develop an imitation learning (IL) enhanced data-driven method to improve the computation efficiency. Specifically, we use a convolutional neural network (CNN) to represent the policies of the slower-scale signal control problem and use the data aggregation method as the learning framework to improve the policies. This algorithm's unique feature is that IL policies' training is based on the optimized results from the model-based multiscale control method. We test the model-based and IL-based methods in simulation, under various traffic scenarios and on multiple networks. We also test the transferability property of the IL-based method by training individual intersection control separately in small networks and applying them to larger networks with various types of intersections. [ABSTRACT FROM AUTHOR]

• This paper proposes the Grid-Based Conflict Index (GBCI) to quantify the collision risks and severity among the vehicles. • This paper introduced a calculation method for the grid-based Post Encroachment Time (PET) and the total kinetic energy change. • This paper proposed a traffic-safety-based AIM model aimed at minimizing the weighted sum of total delay and conflict risk at the intersection. Existing studies on autonomous intersection management (AIM) primarily focus on traffic efficiency, often overlooking the overall intersection safety, where conflict separation is simplified and traffic conflicts are inadequately assessed. In this paper, we introduce a calculation method for the grid-based Post Encroachment Time (PET) and the total kinetic energy change before and after collisions. The improved grid-based PET metric provides a more accurate estimation of collision probability, and the total kinetic energy change serves as a precise measure of collision severity. Consequently, we establish the Grid-Based Conflict Index (GBCI) to systematically quantify collision risks between vehicles at an autonomous intersection. Then, we propose a traffic-safety-based AIM model aimed at minimizing the weighted sum of total delay and conflict risk at the intersection. This entails the optimization of entry time and trajectory for each vehicle within the intersection, achieving traffic control that prioritizes overall intersection safety. Our results demonstrate that GBCI effectively assesses conflict risks within the intersection, and the proposed AIM model significantly reduces conflict risks between vehicles and enhances traffic safety while ensuring intersection efficiency. [ABSTRACT FROM AUTHOR]

The shared autonomous vehicle (SAV), combining shared mobility and vehicle automation, will play a pivotal role in urban transportation. Effectively matching SAVs to stochastic customer demand poses a significant challenge, particularly when addressing microscopic multi-AV coordination to prevent conflicts and congestion within a network. To tackle this issue, we propose a novel traffic control scheme called the Integrated Operation Framework for Order Matching and AV Coordination (I-OFOMAC). This scheme introduces a third-party coordinator responsible for integrating online order matching for the SAV platform and real-time AV coordination for the road manager. Our approach utilizes an offline deep reinforcement learning method to train a state value neural network, which facilitates strategic order matching. Additionally, a neural network approximating bisimulation metrics is trained to enhance exploration and accelerate learning. To achieve scalability in network-level AV coordination, we adopt a scheduled transportation scheme. The coordinator solves the I-OFOMAC during each decision epoch, optimizing order matching and AV coordination. The I-OFOMAC aims to balance the goals of minimizing AV travel delay and maximizing SAV service revenue. We conduct extensive simulation studies in the realistic Wangjing network to evaluate the proposed I-OFOMAC scheme. Compared to two-step benchmark frameworks that implement order matching and AV coordination sequentially, the I-OFOMAC achieves an average increase of 13.28% in platform revenue (ranging from 2% to 22.53%) and a 4.48% reduction in average PAV travel delay (ranging from −6.55% to 20.17%). • Mixed AV zone. • Online operations of joint order matching and AV coordination. • Offline deep reinforcement learning framework. • Bisimulation-guided exploration. • A scalable approximate algorithm. • Numerical studies in realistic Wangjing network. [ABSTRACT FROM AUTHOR]

Traditional traffic signal control is one of the most widely used urban traffic control methods, due to its high efficiency, robustness, and simplicity. However, in the era of connected and automated vehicles (CAVs), it is prospective to enhance traffic control precision and achieve vehicle coordination in a more efficient manner. Focusing on an isolated intersection, this study proposes an innovative concept of Fine-grained Phase (FP) for CAV-enabled traffic control, which realizes more refined right-of-way allocation than traditional signal phases while preserving the simplicity and scalability. The development of FP jointly optimizes traffic control strategy and intersection layout design to fully utilize time-space resources, and multiple forms of FP could be generated in response to various traffic demands; meanwhile, vehicle coordination is achieved in an offline–online manner to balance solution quality and efficiency. Under the FP-based control, each lane is assigned with periodical entry time points using FP design results, then vehicles are supposed to catch up with the appropriate time points to fulfill their trips. Theoretical analyses are conducted under stationary vehicle arrival patterns, where the average delay and the intersection capacity associated with different forms of FP can be analytically acquired. For the implementation of FP-based control, we construct a general offline–online framework, consisting of an offline model that optimizes the geometry of the conflict zone at intersections and the control strategy of vehicles simultaneously, which is formulated as a mixed-integer linear programming problem, and an online FP adoption mechanism that can adapt to varying demand patterns. Under the framework, limited online computation is required, facilitating its practical real-time implementation. Simulation studies under a variety of demand settings show that FP-based control is advantageous over benchmark methods in reducing vehicle delay and also improving intersection capacity in most cases; even for cases with extremely imbalanced demands, FP-based control can admit comparable throughput, while the average delay is much smaller. In addition, results of computation time reveal that FP-based control could realize real-time implementation with negligible computation burden. • An innovative concept called FP for intersection traffic control with CAVs. • Joint optimization of traffic control and intersection layout design. • Offline–online implementation framework to balance solution quality and efficiency. • Theoretical analysis for average vehicle delay and admissible traffic capacity. • Improvement over signalized control and the state-of-art reservation strategy. [ABSTRACT FROM AUTHOR]

The Connected and Autonomous Vehicle (CAV) is an emerging mobility technology that may hold a paradigm-changing potential for the future of transport policy and planning. Despite a wealth of likely benefits that have made their eventual launch inescapable, CAVs may also be a source of unprecedented disruption for tomorrow's travel eco-systems because of their vulnerability to cyber-threats, hacking and misinformation. CAVs manipulated by users, traffic controllers or third parties may act in deceitful ways. This scene-setting work introduces the deceitful CAV , a vehicle that operates in a deceitful manner towards routing and control functionality for 'selfish' or malicious purposes and contextualises its diverse expressions and dimensions. It specifically offers a systematic taxonomy of eight distinctive deceitful behaviours namely: suppression/camouflage, overloading, mistake, substitution, target conditioning, repackaging capability signatures, amplification and reinforcing impression. These as exemplified by their most common attack forms (i.e., starvation, denial-of-service, session hijacking, man-in-the-middle, poisoning, masquerading, flooding and spoofing) are then benchmarked against five key dimensions referring to time frame (short to long duration), engagement (localised to systemic), urban traffic controller infrastructure (single to multiple components), scale (low to high), and impact (low to high). We then suggest mitigation strategies to protect CAV technology against these dangers. These span from purely technological measures referring to the machine-centric triad of vehicles, communication, and control system including adversarial training, heuristic decision algorithms and weighted voting mechanisms to human factor measures that focus on education, training, awareness enhancement, licensing and legislation initiatives that will enable users and controllers to prevent, control or report deceitful activities. • CAVs manipulated by users, traffic controllers or third parties can become deceitful. • The paper introduces a taxonomy of deceitful behaviours: suppression/camouflage, overloading, mistake, substitution, target conditioning, repackaging capability signatures, amplification and reinforcing impression. • These are benchmarked against five key dimensions referring to time, engagement, urban traffic controller infrastructure, scale and impact. • Anti-deception mechanisms consist of technological measures including adversarial training, heuristic decision algorithms and weighted voting mechanisms and human factor measures that focus on education, training, awareness, licensing and legislation. • This is a scene-setting work positioning a diverse traffic control problem and opening up pro-actively an interdisciplinary dialogue. [ABSTRACT FROM AUTHOR]

With the development of autonomous driving and vehicle-to-infrastructure technologies, connected automated vehicles (CAVs) will gradually replace human-driven vehicles (HDVs). However, during the widespread adoption of CAVs, the roads will experience a new type of mixed traffic flow composed of vehicles with different automation levels for an extended period. To analyze the impact of different automation levels on mixed traffic flow, this paper proposes a stability analysis framework for mixed traffic flow that considers different automation levels. The influence of automation level classification on the composition of mixed traffic flow is analyzed first, and the degradation of CAVs in mixed traffic flow is further explored. Then, a stability analysis framework for mixed traffic flow considering different automation levels is proposed, and the impact of key parameters on the stability is analyzed. Finally, simulation experiments are designed to validate the theoretical analysis results. The research findings indicate that (1) the stability of mixed traffic flow improves with the increase in steady-state speed. When vehicle speed exceeds 20 m/s, mixed traffic flow remains stable. (2) When the CAV penetration rate is low, the stability of mixed traffic flow is poor due to the degradation of CAVs. However, with the increase in the penetration rate and automation level of CAVs, mixed traffic flow rapidly tends toward a stable state. (3) Numerous factors influence the stability of mixed traffic flow, with the order of importance being the CAV penetration rate, automation level, headway, and maximum acceleration. Therefore, this study can provide theoretical support for managing and controlling mixed traffic flow with different automation levels in the future. [ABSTRACT FROM AUTHOR]

The emerging technologies in both wireless communication and automation driving that have enabled vehicles to communicate with infrastructure, and with each other, are collectively known as connected vehicle and automated vehicle (CAV) technology. In existing connected automated vehicle-based and traditional traffic control systems, urban arterials with signalized intersections still face issues, including local demand uncertainty and automation level compatibility. Here, a distributed signal control system that integrates automated vehicle path planning information is adapted to address these issues. The proposed method includes planning-based local demand estimation and demand-aware max pressure control in an edge computing environment. A bivariable automation model is developed to represent the various, i.e., L1-L5, automatic driving levels. A real district with typical scenarios is implemented to verify the proposed method. The simulation results show that the proposed demand-aware edge control provides better control results than benchmarks for almost all different performance indices and network demands. The proposed control reduces the waiting time by up to 30%, and the time loss decreased by up to 20%. Moreover, better performances are always obtained at different automation levels, so there is a good compatibility of the automation driving for the adaptation of the CAV flows, especially at the L3-L5 levels. [ABSTRACT FROM AUTHOR]

• Here are the highlights to this paper: • Classify various cyberattaks injection modes. • Propose three car-following models to analyze the cyberattack effects on platoon. • Apply six indicators to comprehensively evaluate driver tolerance, vehicle adaptability, and environmental resistance. The high-level integration and interaction between the information flow at the cyber layer and the physical subjects at the vehicular layer enables the connected automated vehicles (CAVs) to achieve rapid, cooperative and shared travel. However, the cyber layer is challenged by malicious attacks and the shortage of communication resources, which makes the vehicular layer suffer from system nonlinearity, disturbance randomness and behavior uncertainty, thus interfering with the stable operation of the platoon. So far, scholars usually adopt the method of assuming or improving the car-following model to explore the platoon behavior and the defense mechanism in cyberattacks, but they have not considered whether the model itself has disturbance and impact on cyberattack defenses. In other words, it is still being determined whether the car-following model designed can be fully applicable to such cyberattacks. To provide a theoretical basis for vehicular layer modeling, it is necessary to comprehend the self-resistance of different car-following models faced on various cyberattacks. First, we review the car-following models adopted on the vehicular layer in cyberattacks, involving traffic engineering, physical statistics, and platoon dynamics. Based on the review, we divide the malicious attacks faced by the cyber layer into explicit attacks and implicit attacks. Second, we develop a cooperative generalized force model (CGFM), which combines and unifies the r-predecessors following communication topology. The proposed models, labeled the vulnerable cooperative intelligent driver model (VCIDM), the vulnerable cooperative optimal velocity model (VCOVM), and the vulnerable cooperative platoon dynamics model (VCPDM), incorporate the CGFM model and assorted cyberattack injection modes to explain the cyberattack effects on the platoon self-resistance capability. Upon the described models, we provide six indicators in three dimensions from the basic traffic element, including drivers, vehicles, and environment. These indicators illustrate driver tolerance, vehicle adaptability, and environmental resistance when a platoon faces attacks such as bogus information, replay/delay, and communication interruption. We arrange and reorganize the car-following models and the cyberattack injection modes to complete the research on the self-resistance capability of the platoon, which has positive research value and practical significance for enhancing the endogenous security at the vehicular layer and improving the intrusion tolerability at the cyber layer. [ABSTRACT FROM AUTHOR]

• Thirty autonomous vehicle (AV) intersection pre-crash scenarios were identified. • The proportion of AVs being rear-ended by conventional vehicles was 58.38%. • Contributing factors of AV scenarios were identified using association rules. • Key roadway factors were intersection-related area, signal control, and on-street parking. • Insufficient stop, deceleration, and collision avoidance decisions were important causes for AV crashes. Intersections are high-risk locations for autonomous vehicles (AVs). Crash causation analysis based on pre-crash scenarios can provide new insight into these crashes that can lead to effective countermeasures, but there are significant differences in pre-crash scenarios between autonomous and conventional vehicles, and inadequate AV data has put limits on research. The association rule method, however, can yield useful results despite these limits. This study therefore aims to use the method with pre-crash scenarios to understand the characteristics and contributing factors of AV crashes at intersections from the latest 5-year AV crash data. Analysis of 197 AV crashes at intersections revealed 30 types of pre-crash scenarios. The rear-end crash (58.88%) and lane change crash (16.24%) were the most frequently occurring scenarios for AVs. The proportion of AVs being rear-ended by conventional vehicles was 58.38%. The main contributing factors of these two most common AV scenarios were identified by association rules and crash causes were analyzed from the perspective of AV decision-making. The main factors contributing to the AV rear-end scenario were location outside the intersection in the intersection-related area, traffic signal control, autonomous engaged mode, mixed-use or public land, and weekdays, while those for lane change scenarios were on-street parking and the time of 8:00 a.m. Important causes of rear-end crashes attributable to the AV were inadequate stop and deceleration decisions by the AV's automated driving system (ADS) and insufficient collision avoidance decisions in lane change crashes. Identification of the pre-crash characteristics and contributing factors provide new insight into AV crash causation and can be used in the determination of the AV's operational design domain and the development and optimization of the AV's ADS at intersections. These findings can also play a role in guiding traffic safety agencies to discover AV hotspots and propose AV management regulations. [ABSTRACT FROM AUTHOR]

• An internal model-Kalman filtering-based optimal hybrid feedforward/feedback control strategy is developed. • A practically decoupled platoon regulation around an equilibrium state is established. • String stability of the control system thus obtained is analyzed. • Numerical simulations verified the theoretical analysis in decoupling between successive vehicles. This paper presents an internal model-Kalman filtering-based optimal hybrid feedforward/feedback control strategy for traffic platoon control coordination enabled by SAE Automation Level 2 or Level 3 vehicles, i.e., partially automated vehicles (PAVs). Based on the Helly linear car-following model, a PAV platoon is established. Taking each vehicle's characteristic polynomial as the dominant internal model polynomial, an augmented system state model with filtered inputs is formed, and then both the system states and the external disturbance/internal perturbation from previous vehicle can be estimated by a delicately designed Kalman filter. By using a linear quadratic regulation approach, a distributed hybrid optimal feedforward/feedback controller utilizing the local estimated states is constructed to achieve a practically decoupled platoon regulation around an equilibrium state. String stability of the control system thus obtained is further analyzed. Finally, extensive numerical simulations verified the theoretical analysis in decoupling between successive vehicles, in suppressing the influence of various external disturbances, and in maintaining string stability. [ABSTRACT FROM AUTHOR]

• Develop a framework including both stability analysis and implementation constraints for traffic platoon control via sparse autonomous vehicles. • Identify the number of human-driven vehicles that can be stabilized by one single autonomous vehicle. • Establish error dynamics in traffic platoon considering multiple disturbances and error composition. • Propose a novel ring string stability that is flexible and applicable to mixed ring traffic. The emergence of autonomous vehicles (AVs) is expected to revolutionize road transportation and it has been demonstrated that AVs can be employed as mobile actuators to effectively dissipate stop and go waves. This paper proposes a comprehensive theoretical framework including stability and implementation considerations for analyzing traffic platoon control via sparse AVs and discussing the number of HDVs that can be stabilized by one AV. Considering multiple disturbance sources, we first discuss the output error composition and analyze traffic dynamics in a platoon system. Then we discuss the Lyapunov asymptotic stability (LAS) of the system based on eigenvalue analysis and propose a novel ring string stability (RSS), which ensures the boundedness of disturbance error and guarantees that the effect of disturbance will not be amplified over a lap. The proposed RSS is flexible and applicable to mixed traffic, since the string instability for predecessor-follower pairs are allowed, provided they can be compensated by AVs. In addition, the practical constraints including driving safety and vehicle physical limitations are modeled. The worst-case evaluation of constraints is conducted to reproduce the critical scenarios in real life. Finally, we conduct numerical experiments and analyze the traffic platoon control from the aspects of both stability and practical implementations. New findings include: (1) The introduction of AVs is beneficial to dampen the waves and stabilize the HDV flow. (2) Considering the implementation issues, there is an upper bound of HDVs that can be stabilized by one single AV. (3) As the number and amplitudes of exogenous disturbances increase, it will be more challenging for sparse AVs to ensure stability and meanwhile satisfy the practical constraints. [ABSTRACT FROM AUTHOR]

Autonomous driving technology is advancing rapidly, and heterogeneous traffic flow consisting of intelligent controlled vehicles (ICV) and human driving vehicles (HDV) is expected to become the dominant mode on roads. In this study, three driving styles of HDV were analyzed using actual data sets, and the Gipps rule was introduced to establish safety conditions for following and lane-changing rules. A two-lane heterogeneous traffic flow cellular automata model was proposed and simulated using Matlab. According to the simulation results, the impact of ICV penetration rate on traffic capacity is significant, exhibiting a 23.4% to 62.5% increase compared to homogeneous HDV flow within the range of 0.2 to 0.8 ICV penetration rate. Homogeneous ICV flow is about 1.5 times higher than homogeneous HDV flow. Moreover, as the ICV penetration rate increases, there is a gradual decrease in both the lane-changing frequency and congestion level. These findings provide valuable insights into the influence of ICV mixed on traffic flow performance and offer a theoretical basis for managing and controlling heterogeneous traffic flow. • A heterogeneous traffic flow model considering driving styles and intelligent controlled vehicles is proposed. • The improved model effectively replicates the phase states of free, synchronous, and congested flow. • The introduction of intelligent controlled vehicles leads to significant improvements in traffic flow capacity. [ABSTRACT FROM AUTHOR]

The traffic capacity of signalized intersections can be increased using vehicle to infrastructure (V2I) communication by broadcasting traffic control signals to connected autonomous vehicles (CAVs). However, the potential improvement in traffic capacity with only CAVs can be substantially reduced under mixed traffic environments that include autonomous vehicles (AVs) without V2I communication. The main contribution of this work is to increase the traffic capacity (close to that achieved by CAVs with V2I communication) when there is a substantial number of AVs without V2I communication — by using a delayed-self-reinforcement (DSR) approach for vehicle spacing control with already available history of the AVs own control inputs and sensed information. Moreover, analytical estimates are established to predict the improvements in traffic capacity achieved for AVs with the DSR approach compared to the case without the DSR approach. Simulation results match the analytical predictions and show that AVs using the DSR approach (without V2I communication) are able to recover 88% of intersection capacity achieved with CAVs (with V2I communication) and result in 2.7 times the intersection capacity achieved by AVs without DSR on one way traffic. Moreover, similar improvements with DSR are shown for a real-world intersection using the Simulation of Urban MObility (SUMO) platform. AVs with DSR approach are able to recover 96% of intersection capacity achieved with CAVs (with V2I communication) and result in 1.4 times the intersection capacity achieved by AVs without DSR. Improvements using DSR are also seen with mixed traffic that includes human-driven vehicles (HDVs). [ABSTRACT FROM AUTHOR]

• A stochastic dynamic network loading model for the mixed traffic with autonomous vehicles and human-driven vehicles is developed. • The probabilistic link model captures the boundary conditions of a link and approximates the evolution of link state distribution. • The probabilistic node model reflects the between-link dependency by evaluating the expected transmission flow through an iterative algorithm. • Two applications of the proposed model, including a traffic signal control problem and a class-based ramp metering problem, are presented. In this study, we develop a stochastic dynamic network loading (DNL) model for the mixed traffic with autonomous vehicles (AVs) and human-driven vehicles (HVs). The source of stochasticity is the uncertainty inherent in the arrival process of the two classes of vehicular flow. The developed model captures both within-link and between-link traffic flow dependencies and evaluates the network state distribution in an analytical manner. The model has two main components, a probabilistic link model and a probabilistic node model. The link model is a stochastic formulation of the link transmission model (LTM), which captures the boundary conditions of a link and approximates the evolution of link state distribution. The node model, on the other hand, characterizes the flow transmissions across a network node. It reflects the between-link dependency by evaluating the expected transmission flow through an iterative algorithm, with an explicit consideration of the interactions between supply and demand constraints associated with a node. The developed model is validated versus replicated running of the deterministic LTM as well as microscopic traffic simulations, and the results reveal that it yields relatively accurate estimations. We also present two applications of the proposed model, including a traffic signal control problem and a class-based ramp metering problem, to demonstrate its practical value. [ABSTRACT FROM AUTHOR]

The emerging connected autonomous vehicle (CAV) technology offers a significant opportunity to address the congestion problem caused by the selfish routing behavior of human drivers. If the transportation management center (TMC) can control the routing of all vehicles on the network, they could easily reverse this effect. However, during the transition period, CAVs will share roads with human-piloted vehicles (HVs). In the era of mixed autonomy, a more realistic scenario is to route the CAVs to lead the traffic state towards the desired equilibrium. We first consider a repeated routing game in which the TMC can control the routes of CAVs to minimize the system travel time, i.e., CAVs follow the system optimal (SO) routing principle. In this manner, we can influence the travel time traversing each link, which indirectly influences HV users' route choices. As a consequence, HV users may adjust their routes dynamically through day-to-day (DTD) learning to minimize individual travel cost. On the other hand, the SO routing strategy may induce higher travel time for CAVs than HVs within the same origin–destination (OD) pair. To address such inherent unfairness, road pricing is used to influence route choices of HVs to achieve specific supply regulation targets, e.g., traffic restraint. We present qualitative analysis on the stability of the controlled DTD dynamics and the effect of road pricing regarding the unfairness. To the best of our knowledge, this paper is one of the first to design dynamic pricing schemes for a mixed autonomy system under DTD learning dynamics. We carry out numerical experiments to demonstrate the effectiveness of the control schemes. The cooperative CAVs could reduce the total travel time when the market penetration rate of CAVs reaches the lower minimum control ratio that the network could escape the user equilibrium (UE) condition. When the market penetration rate of CAVs reaches the upper minimum control ratio, the network would achieve SO condition even though there are non-cooperative HVs. On the other hand, the pricing scheme can reduce the unfairness index and total travel time of the network when the market penetration rate of CAVs is low. • Day-to-day evolution of the mixed autonomy system. • Routing the CAVs to SO paths to mitigate the impact of selfish routing of HVs. • Tolling the HVs to address the unfairness and to improve the network performance. • Qualitative analysis on the effects of road pricing regarding the unfairness. • Stability guarantee of the day-to-day traffic dynamics under the control schemes. [ABSTRACT FROM AUTHOR]

It has been well documented that the mixed traffic flow of human-driven vehicles (HDVs) and connected and automated vehicles (CAVs) causes an unsound communication environment and weakens the beneficial effect of CAVs. However, the communication heterogeneity in the mixed traffic, which leads to incomplete information flow topology (IFT) and further affects the processes of vehicle control and platoon operation, did not receive its deserved attention. In contrast to previous research related to the IFT and platoons in static size, this study introduced the multi-anticipation and dynamic platoons into a more complex mixture of CAVs and HDVs and jointly considered the mode transition and driver uncertainties, e.g., estimation error and driving compliance. Specifically, we proposed multi-class generic controls to represent the basic longitudinal maneuver for each type of vehicle and developed customized car-following dynamics for each controller. In order to better adapt to the communication heterogeneity in the mixed traffic, we defined a novel controller for CAVs, namely mixed AV-CAV vehicle (MACV) controller and developed the corresponding control algorithm, namely AV-CAV integrated control (ACIC) with multi-source information from both sensors system and inter-vehicle communication. Besides, another important point worth considering in mixed traffic is the spatial distribution of CAVs in platoons. As such, three platoon policies, i.e., the cross policy, the continuum policy, and the random policy, were investigated. To our best knowledge, this is a new attempt to discuss the impact of platoon policies on mixed traffic, with a combination of different IFTs. Finally, multiple traffic flow characteristics, covering robustness, efficiency, safety, and emission, were assessed by calculating their corresponding surrogate measures. Results showed that the proposal of MACV controller and ACIC was expected to efficiently prevent the deleterious degradation from the CAV controller to AV controller and construct a more stable transition for changes in vehicle controls. Furthermore, a high proportion of communicable vehicles could significantly reduce the heterogeneity of mixed traffic and improve traffic flow characteristics. Besides, this study demonstrated the outstanding performance of multi-anticipative IFTs compared to the single-anticipative IFT and revealed the advantage of cross policy in traffic robustness and safety. These findings can help provide feasible solutions for IFTs, platoon operation, and vehicle control in the mixed traffic flow and identify their respective benefits in terms of traffic flow characteristics. • Incorporate multi-anticipation and driver uncertainties into the mixed traffic. • Propose a novel controller for CAVs and develop an integrated control algorithm. • Address dynamic mode transition and platooning in the heterogeneous network. • Construct a framework for comprehensive assessment of traffic flow characteristics. [ABSTRACT FROM AUTHOR]

• Systematic review on urban traffic control under mixed traffic with CAVs. • Cover signal timing, CAV trajectory/route planning, and their cooperative control. • Summarize mixed traffic control methods at intersections with CAV-dedicated facilities. • Outline promising research directions and challenges. Efficient traffic control can alleviate traffic congestion, reduce fuel consumption, and improve traffic safety. With the development of communication and automation technologies, regular vehicles (RVs), connected vehicles (CVs), and connected and automated vehicles (CAVs) will coexist on urban roads in the near future. In this paper, we systematically review the studies on urban traffic control under the mixed traffic environment, which utilize CVs and CAVs to improve the performance of urban traffic control. We first give a glance at the evolution of urban traffic control with the deployment of CVs and CAVs to identify the important and promising research fields in the mixed traffic control. We then review the recent advances in traffic signal timing with CAV/CV trajectory data, CAV trajectory/route planning, and the cooperative control of CAV trajectories and traffic signals from the intersection level to the network level under the mixed traffic environment. Besides, we summarize two types of studies on mixed traffic control at intersections with CAV-dedicated facilities: 1) traffic signal timing and vehicle trajectory planning at intersections with CAV-dedicated lanes, and 2) deployment of CAV-dedicated facilities including CAV-dedicated lanes, links, and zones. Finally, the future research directions and challenges are discussed. We hope that this review could provide researchers with a helpful roadmap for future research on urban traffic control under the mixed traffic environment. [ABSTRACT FROM AUTHOR]

Connected automated vehicles (CAVs) have shown great potential in improving traffic throughput and stability. Although various longitudinal control strategies have been developed for CAVs to achieve string stability in mixed-autonomy traffic systems, the potential impact of these controllers on safety has not yet been fully addressed. This paper proposes safety-critical traffic control (STC) by CAVs—a strategy that allows a CAV to maintain safety relative to both the preceding vehicle and the following human-driven vehicles (HVs). Specifically, we utilize control barrier functions (CBFs) to impart collision-free behavior with formal safety guarantees to the closed-loop system. The safety of both the CAV and HVs is incorporated into the framework through a quadratic program-based controller that minimizes deviation from a nominal stabilizing traffic controller subject to CBF-based safety constraints. Considering that some state information of the following HVs may be unavailable to the CAV, we employ state observer-based CBFs for STC. Finally, we conduct extensive numerical simulations – that include vehicle trajectories from real data – to demonstrate the efficacy of the proposed approach in achieving provably safe traffic. [ABSTRACT FROM AUTHOR]

Urban traffic control is multi-scale in both temporal and spatial domains, which is of great importance to improve the quality and efficiency of our transportation system. In this paper, we propose a general multi-scale modeling and control framework for urban traffic control with connected and automated vehicles (CAVs). We apply the proposed multi-scale framework to explicitly model and solve the two-scale signal-vehicle coupled control (SVCC) problem under the environment of full penetration of CAVs. To solve the SVCC problem, we propose a model predictive control (MPC) scheme and develop a stability analysis method based on the concept of consistency of key states between the two scales. We show numerical results of the SVCC model and comparisons with some benchmark methods. We also discuss how to extend the SVCC model to the mixed traffic flow and tri-scale/multi-scale urban traffic control problems. • Urban traffic control (UTC) is multi-scale in both temporal and spatial domains. • A general multi-scale control framework is developed for UTC with CAVs. • The proposed framework is applied to two-scale signal-vehicle coupled control (SVCC). • A model predictive control (MPC) scheme is developed to solve the SVCC model. • Stability analysis is conducted based on the consistency of key states of the two scales. [ABSTRACT FROM AUTHOR]

While more studies have been focused on adaptive traffic signal control (ATSC) algorithms to learn the control policy from interactions with the traffic environment by using connected and automated vehicle (CAV) data with no human intervention, less mechanism on streamlining the interactions between signal control regulations and CAV-data based traffic features has been integrated into the ATSC algorithms. Such underlying mechanism is of essence to make the CAV-data intelligent functions scalable to varied scales of ATSC system. The presented cyber-physical modeling methodology is attempted to fill in the gap through developing intelligent cell models embedded within the proposed roadside units data processors (RSU-DPs) (i.e., a V2X hub-signalized intersection), which can communicate with on-board units (OBU) mounted on vehicles through C-V2X technologies. The RSU-DPs can directly update the controller for dynamic update of vehicle status and arrivals at the concerned intersection approaches. The intelligent cell model is developed to connect the traffic flow status of the upstream and downstream intersections so as to dynamically adjust or ensure reasonable parameters for signal coordination control (such as offset and/or bandwidth). In this way, the naturalized CAV mobility data can be modeled as a "floating sensing mobility" data source from moving CAVs that will be potentially available ubiquitously on a cycle-to-cycle basis. The simulation-based experiments are designed to test the developed mechanism and models for producing core parameters for implementing the CAV-data-driven self-organized adaptive traffic signal coordination control and testing the functions for improving the efficiency of the corridor mobility, compared with traditional ATSC schemes. [ABSTRACT FROM AUTHOR]

• A lane-level VSL (LVSL) approach with actor-critic framework is proposed to provide differential VSL control. • A hybrid reward function is presented to adjust relationship between the reducing crash risk and improving traffic efficiency. • A twin delayed deep deterministic policy gradient (TD3)-based LVSL approach is established for the first time. • The TD3LVSL shows better performance than baseline models in terms of improving traffic safety and traffic efficiency. Variable speed limit (VSL) control plays a vital role in the emerging connected automated vehicle highway (CAVH) system, which can alleviate recurrent traffic congestion caused by capacity drop and accidents. However, the effect of the VSL on improving traffic efficiency is still controversial. It is necessary to study how to explore the potential benefits of the VSL by balancing its influence on reducing crash risk and enhancing traffic efficiency. To fill the technological gap above, this paper proposes a reinforcement learning-based lane-level VSL (LVSL) control approach to conduct refined traffic control on the mainlines. Firstly, an actor-critic framework is developed to generate and evaluate the discrete speed limits of each lane in continuous action space. To optimize traffic control performance, a hybrid reward function is developed by synchronously considering traffic safety and traffic efficiency of the bottleneck area. Then, to solve the overestimation bias problem of the actor-critic methods caused by function approximation error, a twin delayed deep deterministic policy gradient (TD3) method is introduced to train the framework of the LVSL method. Finally, a real-world recurrent bottleneck of the State Route 91 highway in California is simulated with consideration of the connected automated vehicles to examine the performance of the TD3-based LVSL (TD3LVSL) controller. The simulation results reveal that the proposed method is capable of reducing crash risk and improving traffic efficiency synchronously. Compared with the LVSL controller based on the deep deterministic policy gradient approach, the TD3LVSL controller shows better performance in terms of traffic safety and efficiency. These findings indicate that the proposed controller could contribute to future traffic control in a CAVH environment. [ABSTRACT FROM AUTHOR]

With the popularity of computer internet technology in transportation, connected automated vehicles (CAVs) maintain the traffic flow stability by sharing real-time driving information. However, the open-access environment makes CAVs vulnerable to attacks. Hackers transmit erroneous driving information to drivers, which lead to changes in their driving behavior and cause potential traffic hazards. Based on the summary of existing essays, this paper classifies cyberattacks into three types: false messages, repeat/delay and collusion attack. In addition, mixed flow is a common form of traffic. This paper proposed an extended heterogeneous traffic flow model considering cyberattacks, judgments the stability of the CAVs platoon and designs the controller for stability control. Subsequently, the stability of the new model is analyzed theoretically, and the stability condition of the new model is obtained. The simulation results show that the when CAVs platoon attacked by uncertain types and locations, it can still maintain the stability of the team and reduce the time for the team to return to equilibrium. • An extended heterogeneous traffic flow model considering cyber attacks is proposed. • The stability of the new model is analyzed theoretically. • The controller for stability control for the new model is designed. [ABSTRACT FROM AUTHOR]

The emergence of automated vehicles may have significant impacts on traffic flow. While many studies suggest that fully automated vehicles can improve traffic flow by changing their macroscopic characteristics at different market penetration rates, recent studies show that partially automated vehicles (e.g., adaptive cruise control vehicles) commercially available on the market may negatively impact the traffic flow. With this in mind, existing traffic control strategies, such as ramp metering, may require further modification due to the change of macroscopic traffic flow characteristics in the presence of mixed autonomy traffic. However, the extent to which the ramp metering control algorithm will have to change in response to mixed autonomy traffic is still an open question. In this study, we first formulate the fundamental diagram analytically for mixed autonomy traffic, which is dependent on the market penetration rates of automated vehicles. Next, we model and simulate the composite traffic flow with a mixed autonomy macroscopic traffic flow model, and modify a standard ramp metering control strategy to optimize operations under the new flow conditions. The simulation results show that the total time spent is reduced by 4% compared to the scenario controlled with a standard ramp metering control strategy, suggesting a higher highway throughput is obtained at a merge section when leveraging the modified ramp metering controller. The results also show that the system-wide total delay is reduced by 11% compared to the scenario controlled with a standard ramp metering control strategy, implying the proposed ramp metering control strategy is efficient. • Traffic fundamental diagram depends on the fraction of automated vehicles. • Traffic makeup has potential to degrade traditional ramp metering control. • Ramp metering designed to consider automated vehicles in the flow. [ABSTRACT FROM AUTHOR]

Lane-changing trajectory planning (LTP) is an effective concept to control automated vehicles (AVs) in mixed traffic, which can reduce traffic conflicts and improve overall traffic efficiency. To enhance the lane change safety for AVs, a co-evolutionary lane-changing trajectory planning (CLTP) method is proposed to describe the risk minimization process that co-evolves with the dynamic traffic environment in the limited literature. Firstly, the natural driving data of vehicle trajectory on the expressway provided by the High dataset are used to construct the lane-changing samples. To obtain the future traffic environment information, a deep learning neural network is adopted to capture trajectory dynamics in mobility of surrounding vehicles around a lane-changing vehicle. Secondly, the safe interaction between the subject vehicle and the surrounding vehicles is considered to establish a mathematical model for the temporal and spatial risk identification of a lane change event based on the fault tree analysis method. Subsequently, the risk minimization of lane change is considered as the objective. Based on the acceleration and deceleration overtaking rules and the trapezoidal acceleration method, the longitudinal and lateral displacement schemes during a lane change are designed. Finally, the motion parameters of longitudinal and lateral displacement are acquired to form an ideal lane change trajectory using a genetic algorithm. The results show that this method can effectively achieve higher safety of the lane-changing process, and reduce the traffic conflicts and traffic turbulence caused by dangerous lane-changing behaviors. The findings can provide theoretical support for lane change trajectory planning algorithm design of intelligent vehicles. [ABSTRACT FROM AUTHOR]

Stop-and-go waves are easily caused by unstable traffic due to the collective behavior of human drivers, resulting in higher vehicle fuel consumption and emissions. In this article, we develop a general approach to synthesizing feedback controllers of autonomous vehicles (AVs) for smoothing unstable nonlinear mixed traffic flow, consisting of AVs and human-driven vehicles (HVs). In the context of traffic smoothing and stabilization, prior studies have mainly focused on analyzing string stability of specific car-following models linearized at flow equilibrium points. By contrast, we are interested to analytically synthesize appropriate feedback controllers of AVs for smoothing nonlinear mixed traffic in its general functional forms, covering a broad class of deterministic car-following models commonly seen in the literature. Specifically, by leveraging feedback control theory AVs are controlled to operate in such a way that they closely track a virtual speed profile, i.e., a subtler version of the disturbance resulting from the immediate preceding vehicle. Thus, traffic waves are reduced when propagating backward across controlled AVs. Based on the general functional form of car-following dynamics, we derive a class of effective additive AV controllers that are proven to be able to ensure convergence in tracking desired speed profiles, leading to smoother traffic. In addition, a set of sufficient conditions is devised for guaranteeing car-following safety. More importantly, head-to-tail string stability of the mixed traffic is likely to be achieved with a sufficient market penetration rate (MPR) of AVs. Notably, unlike many existing studies the feedback controllers synthesized for AVs require only local traffic information without having to rely on high degrees of vehicle connectivity, and the rate of smoothing mixed traffic is readily tunable, which is useful for practical implementations. The proposed approach is further illustrated with a theoretical intelligent driver model (IDM) and commercially available adaptive cruise control (ACC) vehicles represented by a well calibrated IDM. Extensive numerical results are presented to show the effectiveness and robustness of the feedback controllers synthesized for AVs on smoothing nonlinear mixed traffic. • Virtual tracking for autonomous vehicles to smooth nonlinear traffic • Applicable to a broad class of deterministic car-following dynamics • Only local traffic information is required for controller synthesis of autonomous vehicles [ABSTRACT FROM AUTHOR]

• A two-layer hierarchical model is developed to coordinate adaptive CAV platoons. • DRL is applied to implement centralized and decentralized control strategies. • The optimal size of CAV platoon is determined by considering multiple objectives. • COOR-PLT has satisfactory convergence performances and the capability of avoiding deadlocks. • COOR-PLT outperforms state-of-the-art methods on reducing travel time and fuel consumption in various traffic conditions. Platooning and coordination are two implementation strategies that are frequently proposed for traffic control of connected and autonomous vehicles (CAVs) at signal-free intersections instead of using conventional traffic signals. However, few studies have attempted to integrate both strategies to better facilitate the CAV control at signal-free intersections. To this end, this study proposes a hierarchical control model, named COOR-PLT, to coordinate adaptive CAV platoons at a signal-free intersection based on deep reinforcement learning (DRL). COOR-PLT has a two-layer framework. The first layer uses a centralized control strategy to form adaptive platoons. The optimal size of each platoon is determined by considering multiple objectives (i.e., efficiency, fairness and energy saving). The second layer employs a decentralized control strategy to coordinate multiple platoons passing through the intersection. Each platoon is labeled with coordinated status or independent status, upon which its passing priority is determined. As an efficient DRL algorithm, Deep Q-network (DQN) is adopted to determine platoon sizes and passing priorities respectively in the two layers. The model is validated and examined on the simulator Simulation of Urban Mobility (SUMO). The simulation results demonstrate that the model is able to: (1) achieve satisfactory convergence performances; (2) adaptively determine platoon size in response to varying traffic conditions; and (3) completely avoid deadlocks at the intersection. By comparison with other control methods, the model manifests its superiority of adopting adaptive platooning and DRL-based coordination strategies. Also, the model outperforms several state-of-the-art methods on reducing travel time and fuel consumption in different traffic conditions. [ABSTRACT FROM AUTHOR]

Intersection control plays a vital role in influencing transportation efficiency inside urban areas. Connected and Automated Vehicle (CAV) technology enables frequent traffic information sharing through vehicular networks, which emerges as a promising way to reduce vehicle traveling time and improve intersection capacity. Meanwhile, with the gradual deployment of CAVs, the traditional traffic flow consisting of human driven vehicles will evolve into a mixed traffic flow composed of traffic participants with differing intelligence capabilities. Considering the evolution towards the mixed traffic environment, we propose a dynamic capacity-aware traffic signal control method: max-pressure for mixed traffic flow (MPMF) traffic signal controller. Firstly, the lane capacity is modeled and approximated based on the saturation flow rate in mixed traffic flow considering the penetration rate of CAVs. Then the dynamic capacity is involved in calculating pressure in the max-pressure method to indicate the importance of a path in mixed traffic flow. A modification strategy for the max-pressure input value is also proposed. An isolated intersection scenario was simulated first to assess the proposed method locally. A multi-intersection network experiment was also conducted to verify the network-level performance of the proposed MPMF method. Comparative results between the proposed MPMF method, the classic max-pressure control, and the existing fixed time control method demonstrate that the MPMF can effectively improve the performance of intersections and be suitable for the multi-intersection road network. [ABSTRACT FROM AUTHOR]

• A data-driven route flow estimation framework is proposed using data from automatic vehicle identification sensors and probe vehicle trajectories. • A prior-information-free objective function is constructed using the notion of flow probe vehicle route penetration rate and route travel time. • The information of observable route flow and observable route travel time is integrated into the estimation model. • The solution uniqueness is guaranteed though an iterative procedure under the entropy maximum principle. Accurate estimation of route flow is crucial to traffic control optimization. Existing route flow estimation models are typically built upon strong assumptions on traveler route choice behavior or turn to data-driven approaches which heavily rely on prior OD estimation and high sensor coverage rate. This study proposes a route flow estimation framework by fusing the probe vehicle trajectory and automated vehicle identification (AVI) data. The notion of probe vehicle route penetration rate (RPR) is utilized for the construction of a novel data-driven objective function. Sensor-OD flow and observable route flow penetration are formulated as model constraints. The solution uniqueness is guaranteed though an iterative procedure under the principle of entropy maximization. Numerical studies are conducted based on pNEUMA, a large-scale full sample trajectory dataset. Results show that the proposed model performs better than the traditional path flow estimation model, especially when the prior flow information is not available. Sensitivity analysis on AVI sensor coverage rate, probe vehicle route coverage rate and RPR distribution demonstrates the proposed model's applicability in practice. [ABSTRACT FROM AUTHOR]

Connected and automated vehicle (CAV) technology is widely recognized as a promising way to improve traffic safety, fuel efficiency and network throughput. However, at the early stage of CAV adoption, there will inevitably be a long period when both CAVs and regular vehicles coexist in the traffic system. To facilitate CAV functionalities in mixed traffic, providing dedicated rights of way for CAVs is necessary. In such scenarios, traffic control requires careful design in the weaving areas that allow CAVs to transfer between dedicated roads and ordinary roads for both safety and efficiency considerations. This paper proposes a dynamic programming approach for the coordinated trajectory planning of CAVs in mandatory lane-changing scenarios, in which CAVs have to switch from a dedicated CAV lane into parallel ordinary lanes with mixed traffic. For a group of CAVs that are poised to enter the ordinary road, the proposed method aims to provide each CAV with a suggested lane-changing gap and a reference trajectory in a coordinated manner. To solve the problem, we establish a dynamic programming formulation to determine the suggested gaps for all CAVs. We formulate a nonconvex quadratic-constrained quadratic programming (QCQP) problem to characterize the trajectory determination problem, and a relaxation-based approximate algorithm is developed to solve the QCQP. Simulation results show that, compared to the benchmark methods, the proposed method offers substantial benefits both in terms of traffic efficiency and in-vehicle passengers experiences. • Planning of lane-changing from dedicated CAV lanes to non-cooperative HDV traffic. • Coordinated optimization toward efficient and smooth lane-changing trajectories. • Novel algorithm to decompose optimization problems based on dynamic programming. [ABSTRACT FROM AUTHOR]

• Three dynamic speed limit strategies are proposed to reduce freeway collision risks. • Impacts of CAV distribution patterns in mixed traffic flow on control strategies are investigated. • Extensive simulation experiments are conducted to examine effects of proposed control strategies. Connected and automated vehicles (CAVs) have the great potential to improve traffic flow because of their particular characteristics of connectivity and automation. This study aims to develop CAV control strategies based on car-following speed balance, which are defined as dynamic speed limit (DSL) strategies, and examine their performances on reducing freeway collision risks via microscopic simulations. The core idea of DSL strategy is to command the CAVs to slow down actively before reaching the bottlenecks, and form moving barriers to guide the following human driven vehicles to passively decelerate. Three DSL strategies are first developed for CAVs based on vehicle dynamics principles, and the influences of various position distribution patterns of CAVs on three strategies are compared in the one-lane scenario. Then, the DSL strategy with the best performance is selected based on simulation experiments, and a conventional variable speed limit control is used to compare the performance of our proposed methods. Finally, the DSL strategy based on Min control is tested in the two-lane, three-lane and four-lane scenarios to verify the effectiveness. Simulation results indicate that: (1) three DSL strategies based on CAVs can significantly reduce collision risks when CAVs reach a certain proportion; (2) the uniform distribution of CAVs can maximize the effect of the moving barriers; (3) DSL strategy based on Min control is negatively affected by lane-changing behaviors, but still works well in the multi-lane scenario. [ABSTRACT FROM AUTHOR]

Autonomous vehicles (AVs) are emerging in the automobile industry with potential benefits to reduce traffic congestion, improve mobility and accessibility, as well as safety. According to the AV collision data managed by the California Department of Motor Vehicles (DMV), however, the safety issue of AVs has continuously been a concern. This paper aims to learn the contributing factors to AV crash severity from the latest 3-year AV collision data. To achieve the objective, we develop an AV crash severity classification tree with the possible contributing factors by the cost-sensitive classification and regression tree (CART) model, which can deal with the class imbalance issue raised from the AV collision dataset. Our results show that the main factors affecting AV crash severity level include manufacturer, facility type, movement preceding collision, collision type, light condition and year. These findings could provide useful insights for traffic engineers or AV manufacturers to raise effective counter measures or policies to mitigate AV crash severity. • Identify the contributing factors to autonomous vehicle (AV) crash severity • Tackle the class imbalance issue of AV crash dataset with cost-sensitive CART method • Analyse the effect of different class-weighted ratios • Compare the classification results of cost-sensitive CART model with other methods • Provide implications on AV crash severity mitigation and safety improvement [ABSTRACT FROM AUTHOR]

In this paper, the feed-forward control effect is introduced into the car-following system, which combines the proportional differentiation and velocity feedback effect to form a new compound compensation method to improve the stability and rapidity of the traffic flow system. The unit step input response in time domain analysis method is used to analyze the system and the feed-forward effect of the system. Using the small gain theorem and the Rouse criterion, we obtain the stability conditions of the improved system and obtain the neutral stability curves. Next, setting up simulation experiments discuss the influence of feed-forward effect on autonomous traffic flow system. The results show that the introduction of feed-forward effect can effectively improve the rapidity of traffic flow system responding to disturbances. Combined with appropriate proportional differentiation and velocity feedback, the system can be both stability and rapidity, which is meaningful for autonomous driving. • The feed-forward control strategy is introduced into the autonomous driving traffic flow control to form a new compound compensation control method. • According to the basic principle of control theory, the stability of traffic flow system is analyzed qualitatively and quantitatively by using time-domain analysis and small gain theorem. • Simulation experiments are set up to analyze the control effect of feed-forward control strategy on autonomous traffic flow. [ABSTRACT FROM AUTHOR]

• The real-world data is used to understand the effects of the leading automated vehicles on the following human drivers. • Drivers show lower driving volatility, shorter time headways and higher time-to-collision values when following automated vehicles. • Drivers' heterogeneous car-following behaviors when following automated vehicles under different traffic environments are identified. As the market penetration rate of automated vehicles (AVs) increases, there will be a transition period when the traffic stream is composed of both AVs and human-driven vehicles (MVs) in the near future. However, the interactions between MVs and AVs, especially whether MVs will behave differently when following AVs compared to following MVs, have not been fully understood. Previous studies in this field mainly conducted traffic/numerical simulations or field experiments to investigate human drivers' behavior changes, but these approaches all have critical drawbacks such as simplified driving environments and limited sample sizes. To fill in the knowledge gap, this study uses the high-resolution (10 Hz) Waymo Open Dataset to reveal differences in car-following behaviors between MV-following-AV and MV-following-MV cases. Driving volatility measures, time headways and time-to-collision (TTC) are adopted to quantify and compare MV-following-AV and MV-following-MV interactions. The principal component analysis (PCA) is applied on the high-dimensional feature space, followed by the hierarchical clustering on the dimension-reduced feature set to categorize MV driving styles when following AVs. The comparison results indicate that MV-following-AV events have lower driving volatility in terms of velocity and acceleration/deceleration, smaller time headways and higher TTC values. Furthermore, the clustering results reveal that human drivers when following AVs exhibit four different car-following styles: high-velocity-non-aggressive, high-velocity-aggressive, low-velocity-non-aggressive, and low-velocity-aggressive. These findings highlight the vital importance of taking into account the heterogeneity of MV-following-AV behaviors when designing mixed traffic control algorithms and can be beneficial for AV fleet operators to improve their algorithms. [ABSTRACT FROM AUTHOR]

Control of mixed traffic, consisting of manual driving vehicles and automated driving vehicles, remains a major challenge. In this paper, a novel pinning control strategy for mixed platoon to achieve indirect control of manual driving vehicles is proposed. Besides the control algorithm of pinning nodes, the impact of the spatial distribution of automated driving vehicles on traffic is studied. The approach is compatible with the existing control of automated driving vehicles in mixed vehicle platoon. Numerical simulation results illustrated the effectiveness of the proposed pinning control strategy. • A new cyber–physical description of mixed platoon is proposed. • A novel pinning control framework for mixed traffic is given. • The impact of the spatial distribution of automated driving vehicles on mixed platoon is studied. [ABSTRACT FROM AUTHOR]

• Propose a cooperative signal-free intersection control framework under V2X communications. • Proposed framework integrates virtual platooning control and traffic flow regulation. • Virtual platooning controller mitigates effects of parametric inaccuracies and unparameterized disturbances. • Virtual platooning controller guarantees string stability. • Traffic flow regulation alleviates traffic spillbacks and travel delays. The emerging technologies of connectivity and automation enable the potential for signal-free intersection control. In this context, virtual platooning is posited to be an innovative, decentralized control strategy that maps two-dimensional vehicle movements onto a one-dimension virtual platoon to enable intersection operations. However, the effectiveness of virtual platooning-based control can be limited or degraded by parametric inaccuracies and unparameterized disturbances in vehicle dynamics, heavy traffic congestion, and/or uncoordinated platoons in multi-lane intersections. To explicitly address these limitations, this study proposes a hybrid cooperative intersection control framework consisting of microscopic-level virtual platooning control and macroscopic-level traffic flow regulation for traffic environments with connected autonomous vehicles. In virtual platooning control, vehicles approaching an intersection are organized into coordinated independent virtual platoons to avoid potential conflicts triggered by platoon formation changes. Through coordination, vehicles in a platoon are grouped into compatible passing sets to maintain desired safe spacing when proceeding through the intersection. We propose a distributed adaptive sliding mode controller (DASMC) which uses the backstepping control method and model reference adaptive control method to address parametric inaccuracies, and the sliding mode control method to consistently suppress the negative effects of the unparameterized disturbances. Each vehicle approaching the intersection utilizes the kinematic information from neighboring vehicles to implement the DASMC in a distributed manner such that vehicles within the same virtual platoon can achieve consensus safely. However, virtual platooning control cannot preclude excessive traffic from approaching the intersection, which can cause undesired spillbacks and degrade intersection control performance. To address this issue, traffic flow regulation is integrated with the virtual platooning control using an iterative feedback loop mechanism. In each iteration of the iterative feedback loop, a constrained finite-time optimal control (CFTOC) problem is solved to determine the optimal input flow permitted to proceed through the intersection, and the virtual platooning control provides feedback on the queue status to the CFTOC to initiate the next iteration. The effectiveness of the proposed intersection control framework is evaluated through numerical experiments. The results indicate that the proposed virtual platooning DASMC controller can mitigate the effects of parametric inaccuracies and unparameterized disturbances to achieve consensus for approaching vehicles, as well as guarantee string stability. Further, the proposed framework can alleviate traffic spillbacks and travel delays effectively through traffic flow regulation. [ABSTRACT FROM AUTHOR]

Heterogeneity is widely observed in various aspects of urban transport, such as road types, vehicle dynamics, and demand patterns, which greatly complexifies transport systems and hinders the successful control and management of traffic. On one hand, intelligent strategies are required to relieve the loss of efficiency caused by the mixed traffic flow composed of vehicles with different speeds; on the other hand, the spatial heterogeneity of travel demand distribution requires pertinent control schemes to improve the network-level mobility and the phenomenon of oversaturation under heavy traffic also demands an appropriate way to resolve. Leveraging the advantages of connected and automated vehicle (CAV) technology, this study proposes a multi-rhythm control (MRC) scheme to accommodate heterogeneous network traffic in a CAV environment with 100% penetration rate. While the rhythmic control (RC) proposed by (Lin et al., 2021) could organize traffic efficiently and in an orderly manner, the multi-rhythm strategy has greater potential to not only improve the efficiency of heterogeneous traffic by the design of different virtual platoons (VPs) catering to different vehicle types, but also address the spatial heterogeneity of network traffic by implementing differentiated cycle lengths among the road network. For the optimal design of the MRC scheme, a two-step model is developed to minimize the total travel cost, where the first step focuses on the mesoscopic level to jointly optimize the multi-rhythm scheme, VP design and lane configuration for both undersaturated and oversaturated cases, while the second step focuses on the microscopic design to optimize the space–time trajectories of VPs under the premise of collision free. Numerical examples and simulation experiments are conducted to test the performances of the MRC scheme compared to a naive version of RC with a homogeneous rhythm and a traffic signal control (TSC) strategy. The results show that among all control strategies, the MRC scheme has the best performance under all kinds of demand patterns, which is reflected in both the lowest travel cost and the highest network throughput. • A multi-rhythm control strategy to accommodate heterogeneous network traffic. • Different kinds of virtual platoons to enable compatible movements for vehicles. • A two-step model to solve the optimal design problem of MRC scheme for various saturated cases. • Simulation experiments to validate the performance of the MRC scheme. [ABSTRACT FROM AUTHOR]

• The supervised learning algorithm based on XGBoost is exploited to classify risk levels of interactive vehicles in real-time during the merging behaviour of mandatory drivers; • The Bayesian hierarchical model comprising vehicle physical state layer, multi-vehicle interaction layer and risk probability layer is used to predict the probability of risk occurrence under different risk levels; • The probabilistic-model parameters of Bayesian hierarchical are calibrated using MCMC Gibbs sampling method; • The K -Fold cross-validation method and posterior prediction of P -value are used to validate the proposed model of risk level and the probabilistic-model, respectively. • The case study is conducted to present safety estimation results of quantitative risk for CAVs through a few merging cases. The risky lane-changing manoeuvre of vehicles often occurs at expressway entrances, which would result in a high crash risk in the freeway system and significantly impact its safety. The highly anticipated environment of connected and autonomous vehicles (CAVs) is expected to reduce the associated crash risk of lane changing by offering various types of driving support, which utilise surrounding traffic information. The modelling crash risk under the environment of CAV driving during mandatory lane changing in merging areas faces new challenges due to the novelty of CAVs and subsequent shortage of data. To explore such risk situation of multi-vehicle interaction at expressway entrances, this study proposed a supervised learning algorithm and a Bayesian hierarchical model to assess risk levels and predict the probability of risk occurrence at different risk levels of interactive vehicles in real time of mandatory driving behaviour during the merging process. The learning algorithm, based on XGBoost, was exploited to classify risk levels. The Bayesian hierarchical model was used to analyse the probability of real-time risk comprising vehicle physical state layer, multi-vehicle interaction layer and risk probability layer. The probabilistic model parameters were calibrated using Markov Chain Monte Carlo (MCMC) Gibbs sampling method. The K -fold cross validation method was used to validate the proposed model of risk level. The probabilistic model validity was tested through posterior prediction of P -value. The quantitative risk estimation of CAVs through a few merging cases was conducted. Results show that the identification accuracy of slight, low, moderate and high risk is 94.24%, 85.82%, 84.16% and 79.69%, respectively. The P -value of Durbin–Watson's posterior, normal hypothesis, test distribution symmetry and kurtosis are all close to 0.5. Therefore, the method of real-time risk assessment is convergent and has good fitting. This research can promote cautious driving behaviours and provide reference for driver's decision making in the long term under the environment of CAVs. [ABSTRACT FROM AUTHOR]

• Develop a three-level model to integrate signal timing and CAV trajectory planning. • Left-turn and through CAVs share CAV-dedicated lanes with trajectory planning. • CAVs cross the intersection during shared phases with HVs. • Propose a repeated-barrier-group cycle structure in signal timing. • Design a rolling-horizon scheme to dynamically apply the model with varying traffic. Connected and automated vehicles (CAVs) and human-driven vehicles (HVs) are expected to coexist in the near future. CAV-dedicated lanes and phases have been explored to handle the uncertainty in the driving behavior of HVs in the mixed traffic environment. However, CAV-dedicated phases could significantly sacrifice HV benefits. This study proposes a shared-phase-dedicated-lane (SPDL)-based traffic control model at an isolated intersection with CAV dedicated lanes in the mixed traffic environment. In the proposed model, left-turn and through CAVs share CAV-dedicated lanes and cross the intersection during the shared phases with HVs. A three-level optimization model is developed. At the upper level, a standard NEMA (National Electrical Manufacturers Association) ring barrier structure is used for the signal optimization and barrier durations are optimized by dynamic programming to minimize the total vehicle delay. At the middle level, phase sequences and phase durations are optimized by enumeration for the given barrier from the upper level and the minimum vehicle delay is fed to the upper level. At the lower level, CAV platooning in the buffer zone and trajectory planning in the passing zone are conducted based on the signal timings of the barrier from the middle level and the travel time of CAVs is fed to the middle level. A rolling-horizon scheme is designed for the dynamical implementation of the proposed model with time-varying traffic conditions. The proposed model is applicable to intersections with more than two lanes in each approach and without shared lanes by left-turn and through HVs, which are commonly seen critical intersections in urban traffic control. Numerical studies validate the advantages of the SPDL-based control over the blue-phase-based control in previous studies in terms of average vehicle delay and intersection capacity. Further, the SPDL-based model is extended to serve as an alternative approach without the buffer zone. [ABSTRACT FROM AUTHOR]

• This paper investigates the key ways for automated vehicles (AVs) to improve the traffic efficiency of two-dimensional urban network traffic. • Different cooperative driving strategies are investigated in a systematic and comprehensive way, and we find that the passing order has a dominant impact on the network traffic efficiency. • This paper compares the differences and characteristics of the impact of the passing orders and the car-following gaps on the urban network traffic. • By using the macroscopic fundamental diagram (MFD) as the performance metric, we find that a better passing order can broaden the free flow region and improve the maximum flow, which is of great benefit to the network traffic efficiency. Cooperative driving, especially the passing order, is the critical link to improve the traffic efficiency of the road network by using automated vehicles (AVs). However, most studies have only considered the performance of the passing order at the isolated intersection and have not yet investigated its impact on the road network. In this paper, we will focus on the performance of the passing orders derived from different cooperative driving strategies on the network traffic through a series of simulation experiments. Meanwhile, we will compare the impacts of the passing order at intersections and the car-following gap in straight links on the network traffic efficiency. The experiments results show that the passing order has a dominant impact on the network traffic efficiency, and a better order can significantly raise the curve of the macroscopic fundamental diagram (MFD); due to the inevitable conflicts in the two-dimensional traffic, the choice of the car-following gap within a reasonable range has a relatively small improvement on the network traffic efficiency. The findings in this paper have instructive significance for the rising research on network-wide cooperative driving and provide a systematical perspective for network traffic control. [ABSTRACT FROM AUTHOR]

Connected and automated vehicles (CAVs) promise to significantly improve road traffic. To a certain extent, this situation is similar to the expectations at the end of the last century about the positive effects that the introduction of Adaptive Cruise Control (ACC) systems would have had on motorway traffic. The parallelism is interesting because ACC equipped vehicles represent the first level of vehicle automation and are now widely available on the market. In this light, studying ACC impacts can help to anticipate potential problems related to its widespread application and to avoid that AVs and CAVs will lead to the same problems. Only a few test-campaigns had been carried out studying the ACC impacts under real-world driving conditions in quantitative terms. To bridge this gap, the Joint Research Centre of the European Commission has organized a number of experimental campaigns involving several ACC-equipped vehicles to study different implications of their widespread. In this context, the present paper summarizes the outcomes of a test campaign involving 10 commercially available ACC-equipped vehicles. The test campaign has been executed in two different test-tracks of the ZalaZONE proving ground, in Hungary. The tests have been carried out at low-speeds (30–60 km/h) and have involved platoons of vehicles of different brands and different powertrains, which were tested in a variety of vehicle orders and with different settings of their ACC systems. Test results have been used to derive information about the properties of the different ACC systems, to study their string stability, to study the effect of ACC systems on traffic flow, and to draw inference about the possible implications on energy consumption and traffic safety. Results confirm the previous findings in terms of string instability of the ACC and highlight that in the present form, ACC systems will possibly lead to higher energy consumption and introduce new safety risks when their penetration in the fleet increases. However, they also highlight that the materialization of the above findings for AVs depends on the operational logic that manufacturers will adopt during the implementation phase. Therefore, results suggest that functional requirements to guarantee string stability and in general to not disrupt the normal flow of traffic should be introduced both for ACC and for any automated system that will be placed on the market in the future. [ABSTRACT FROM AUTHOR]