WATCH-OVER is a European project, aiming at the enhancement of road safety and the impairment of traffic accidents involving vulnerable road users (VRUs), such as pedestrians, bicyclists and motorcyclists, in urban and extra-urban areas. The project carries out research and development activities, in order to design an integrated cooperative system for accident prevention. In this paper, the concept of the Human Machine Interface of the WATCH-OVER system is discussed and its user-centred approach, based on a user requirement survey, is described. Regarding the HMI, the basic functionalities and elements, as well as the preliminary guidelines that endorse the WATCH-OVER system approach, are presented.