Conventional cruise control systems fulfil the function of automatic speed control. A desired speed is selected by the driver, and a control system operates on the throttle to maintain this desired speed. When traffic density is moderate or high, the driver is faced with having to adjust the set speed regularly in order to maintain a comfortable distance from preceding vehicles and will frequently have to brake, disengaging the cruise control. Thus conventional cruise control can become a source of irritation when used in moderate or heavy traffic. If a distance sensor is added to a conventional cruise control system, then it is possible to add distance keeping to the basic speed control function. This forms the basis for adaptive cruise control, which can be further improved if a limited authority braking system is incorporated. Use can then be made of both throttle and brake actuators to control the distance and relative velocities between a vehicle and a preceding target vehicle.