Nowadays, there is no doubt that the implantation on a massive scale of safe autonomous driving is very difficult using only the information coming from the ego-vehicles, which are subject to the limitations of their visual horizon. For autonomous driving systems to become a reality, it is essential to provide the system with two fundamental elements: connectivity and cooperative services. However, both elements are still at a very early stage of their development, both in communications technology and in the organization and generation of support information. In addition, added difficulties are presented, such as trans-national barriers to accessing services and exchanging information with other vehicles and infrastructure. This paper presents the work done within the European project AUTOCITS, where three cooperative, connected and autonomous driving pilots have been carried out in three cities belonging to the trans-European Atlantic Corridor: Madrid, Lisbon and Paris. Within these pilots, the results of cross-border interoperability are described in detail, both at the communications and at the autonomous driving level. Also, results are analyzed in order to have a set of recommendations to ensure a successful deployment of cooperative, connected and autonomous driving (CCAD) at European level