A method of generating a horizon for use in an Advanced Driver Assistance System (ADAS) of a vehicle using stored digital map data, wherein the digital map data comprises a plurality of segments representative of roads of a road network, said method comprising: determining a segment representing a road on which the vehicle is currently travelling based on an identified current location of the vehicle; using the determined segment to identify an upcoming decision point of the road network from the digital map data; determining an incoming path to the decision point with respect to which outgoing paths are to be defined; identifying if the decision point is a roundabout from the digital map data, and, when the decision point is determined to be a roundabout, treating the roundabout as a single junction such that the outgoing paths (p1, p2, p3, p4) of the decision point correspond to exits of the roundabout; determining data indicative of the relative probability that each of a plurality of possible outgoing paths (p1, p2, p3, p4) associated with the decision point will be taken by the vehicle using one or more of stored digital location-based data including digital map attribute data, vehicle data and driver data, wherein each segment of the digital map data is associated with data indicative of one or more attributes of the road represented by the segment, wherein the digital location-based data used to determine the relative probability data comprises the attribute data, and wherein the attribute data includes data indicative of: a geometry of the road; a gradient of the road; the angle between the road corresponding to the possible outgoing path and the road corresponding to the incoming path; a road class of the road; a speed limit associated with the road; vehicle flow data indicative of vehicle flow along the road; and vehicle speed profile data for the road; wherein the vehicle data includes vehicle type; vehicle speed; historical movements of the vehicle, such as a turn history of the vehicle; and frequency of travel of the vehicle along a given segment of a road network; and wherein the driver data includes data indicative of the past behaviour of the driver, such as a turn history of the driver; determining one or more predicted paths that the vehicle will be expected to travel in the immediate future at the decision point using the determined relative probability data; and generating the horizon using the prediction of a most probable outgoing path and at least one alternative outgoing path. determining a segment representing a road on which the vehicle is currently travelling based on an identified current location of the vehicle; using the determined segment to identify an upcoming decision point of the road network from the digital map data; determining an incoming path to the decision point with respect to which outgoing paths are to be defined; identifying if the decision point is a roundabout from the digital map data, and, when the decision point is determined to be a roundabout, treating the roundabout as a single junction such that the outgoing paths (p1, p2, p3, p4) of the decision point correspond to exits of the roundabout; determining data indicative of the relative probability that each of a plurality of possible outgoing paths (p1, p2, p3, p4) associated with the decision point will be taken by the vehicle using one or more of stored digital location-based data including digital map attribute data, vehicle data and driver data, wherein each segment of the digital map data is associated with data indicative of one or more attributes of the road represented by the segment, wherein the digital location-based data used to determine the relative probability data comprises the attribute data, and wherein the attribute data includes data indicative of: a geometry of the road; a gradient of the road; the angle between the road corresponding to the possible outgoing path and the road corresponding to the incoming path; a road class of the road; a speed limit associated with the road; vehicle flow data indicative of vehicle flow along the road; and vehicle speed profile data for the road; wherein the vehicle data includes vehicle type; vehicle speed; historical movements of the vehicle, such as a turn history of the vehicle; and frequency of travel of the vehicle along a given segment of a road network; and wherein the driver data includes data indicative of the past behaviour of the driver, such as a turn history of the driver; determining one or more predicted paths that the vehicle will be expected to travel in the immediate future at the decision point using the determined relative probability data; and generating the horizon using the prediction of a most probable outgoing path and at least one alternative outgoing path.