Microscopic control and optimization of connected and automated vehicles at highway on-ramp merging areas: a comprehensive review

Highway ramp merging areas are critical bottlenecks affecting traffic safety and operational efficiency, where interactions between mainline and ramp vehicles often induce traffic conflicts, crashes, and delays. Connected and automated vehicles (CAVs), enabled by precise trajectory control and real-time cooperation, provide a promising solution for improving ramp merging operations. Existing reviews mainly compare algorithms or application scenarios, with limited attention to the physical process of ramp merging, in which longitudinal speed and spacing adjustments must be coordinated with lateral lane-changing motion to enter a safe mainline gap within a limited merging area. To examine how this motion is planned and realized, this paper develops a decision-planning and control-execution framework for reviewing CAV ramp merging strategies. The two functional modules are analyzed across individual-vehicle, general multi-vehicle, and platoon-based forms of vehicle organization, focusing on their technical methods, implementation, and effects on traffic performance. Their system-level organization is then compared under hierarchical cooperative, integrated, and hybrid control architectures. Finally, future research directions for CAV ramp merging control are identified.