This paper discusses the design of an Advanced Driver Assistance System (ADAS) that ensures safety when traffic rules are respected by all road users. This concept, referred to as legal safety, is proposed as a basis that permits human and automated drivers to share the road infrastructure. It is illustrated for a Highly Automated driving System with speed keeping, distance keeping and lane changing functionalities on highways (HAS). The requirements legal safety places upon HAS components are presented, with a special focus on the co-pilot which calculates a safe trajectory for the vehicle based on perception of lanes and traffic signs and prediction of the trajectories of objects in the environment. A lane coordinate system is proposed as a powerful reference for the trajectory calculations of the co-pilot. The system controls the vehicle and communicates with the driver, according to an automation mode scheme inspired by the horse-rider metaphor (H-metaphor).