The road vehicles have on board communication facilities on a vehicle to vehicle basis and also on a vehicle to roadside unit basis (C1,C2). The vehicle has a controller (M2) that is coupled (M1) to a steering (M3) and speed (M4) control modules. When a vehicle is about to enter a main road from such as a slip road the communication operates to determine the point of intersection and adjusts the speed so that a safe distance relative to other vehicles is maintained.
