This thesis provides a theoretical framework for the arise of stop-and-go waves in platoons of Human-driven Vehicles (HVs) on a ring road topology. Possibilities and limitations for traffic control and suppression of such waves via a single Autonomous Vehicle (AV) are considered. Being HVs prone to show string instability, i.e. a type of instability caused by a disturbance acting on one of the vehicles and which is amplified throughout the platoon, the analysis starts from the observation that the standard notion of string stability on a ring road is too demanding for a mixed (human and automated) traffic scenario. Thus, after providing a new definition of string stability on a ring roadway, two directions are pursued. First, only HVs on a ring roadway are considered and their stability/instability analyzed. Second, a mixed-platoon with a single AV and the remaining HVs is placed on the ring and the AV is designed in order to ensure equilibrium stability. By means of this investigation, possibilities and limitations of traffic control via a single AV are emphasized.