# Cooperative Roundabout Control Strategy for Connected and Autonomous Vehicles
## 1 Introduction
In summary, the lack of precision in human drivers' driving behavior can weaken roundabouts' safety advantage, while the oversimplified cooperation rules compared to adaptive traffic signal control scheme can limit the system performance and the application of roundabouts in high-demand situations. Advances in communication and automation technologies have led to the development of connected and autonomous vehicles (CAVs), which can enable accurate vehicle operations and comprehensive cooperation through vehicle-to-vehicle (V2V) communications. CAVs can address the aforementioned limitations of roundabouts in the context of human-driven vehicles and thereby illustrate the advantages of roundabouts over signalized intersections. Existing studies have shown that CAVs are able to enhance system performance with safety guarantees by incorporating additional real-time information collected using V2V communications into comprehensive control schemes [1; 7]. In this context, cooperative adaptive cruise control (CACC) could be leveraged to ensure CAVs' safety while allowing them to pass through smaller-radius roundabouts at higher speeds. The connectivity ability further enables holistic cooperative control strategies to enhance system performance. Several studies have developed model-based [8; 9] or machine learning-based [10; 11] control strategies for traditional intersection geometries without using traffic signals, which are labeled as signal-free intersection control strategies, to reduce traffic oscillations caused by signalized intersections and enhance smoothness. However, the \"cross\" flow pattern within the intersection remains the same as in signalized intersections, thereby retaining the safety issues caused by crossing conflicting points. For safety reasons, more constraints are incorporated in signal-free intersection control models, which further constrains the enhancement of system performance under CAV technologies. While roundabouts can potentially enable better performance in a CAV environment compared to signal-free intersections, few efforts have focused on developing roundabout control strategies. Ref. [12] proposed a fuzzy logic steering control for autonomous vehicles in roundabouts. Ref. [13] developed a parameter-based path generation method for autonomous vehicles in roundabouts. However, these studies focus on the control strategy for a single-vehicle scenario. Other studies [14; 15] proposed game-theoretic-based decision-making methods for a two-vehicle scenario in a roundabout. Hence, existing studies address the control problem for specific scenarios and cannot be generalized to high-demand intersections. Moreover, they mainly address safety considerations. However, demand is typically higher at intersections in urban networks, which entails the need for system performance (i.e., smoothness, total delays, and throughput) analysis for intersection control strategies. This motivates the need for a systematic approach to leverage the advantages of roundabouts in the CAV context. This study seeks to explore the following two research questions. How can CAVs leverage emerging technologies when traveling through intersections with the circular flow pattern to enhance efficiency and safety? Does the circular flow pattern have advantages over the \"cross\" flow pattern in future CAV environments? The answers to these questions can shed light on future traffic operations and infrastructure design at intersections. Therefore, this study proposes a cooperative roundabout control strategy for CAVs to exploit the advantages of roundabout design to promote performance efficiency of roundabouts while ensuring vehicle safety. As shown in Figure 1, the proposed model is a hierarchical framework that consists of a roundabout flow control model, a merge-in decision model, and a within-roundabout virtual platoon control model. The time horizon of interest is divided into equal time intervals within which the incoming demands of the roundabout are assumed constant.