Managing connected and automatic vehicles (CAV) in road networks is a major challenge with significant scientific and practical relevance. The mixed traffic scenario is one issue that necessitates special attention during the introductory phase of CAVs. This paper targets to improve the social acceptance of CAVs, to reduce the travel delays and to deliver undisturbed automatic drivings through forming regulated CAV platoons. Furthermore, a strategy that operates CAVs to drive in platoons which are led by manually driven CAVs is proposed and three specific approaches are designed to utilize the strategy. With an overall objective of maximizing the platoon size, these approaches determine the role of each CAV over time with formulating different constraints on the CAV role switch. Specifically, the switch moment is constrained in the first approach; apart from the moment constraint, the switch area is restricted in the second approach; whilst the third approach has an additional constraint that only allows specific CAVs to be platoon leaders. Their performances are measured by travel time delay, the switch frequency between roles and the automated driving duration. The results demonstrate that the first approach outperforms others with a low CAV penetration level and the third approach can increase the undisturbed automated driving duration without significantly increasing the total travel delay with a high CAV penetration level.