output an at control unit 13 controls a gear position of an automatic transmission it sends information on the subject vehicle speed and the at gear position to the acc control unit 2 over the communication line this allows the acc control unit 2 to perform a constant speed control or an auto cruise and vehicle to vehicle distance control an adaptive cruise control the constant speed control auto cruise means to maintain a vehicle speed set by a driver for the subject vehicle the headway distance automatic control on the other hand means to maintain a predetermined headway distance from the forward vehicle when it becomes shorter as a result of the forward vehicle 39 s running at a speed lower than the set vehicle speed of the subject vehicle fig2 is a control block diagram of the acc control unit 2 the acc control unit 2 receives as inputs thereto information on a brake lamp switch 23 a subject vehicle speed 13 an at gear position 13 a headway distance from the forward vehicle 1 a relative speed 1 a yaw rate 1 a master cylinder pressure 8 a throttle opening signal a throttle angle signal an acc main switch 3 an acc related set switch and signals 22 24 25 based on these pieces of information the acc control unit 2 transmits a torque command value to the engine control unit 12 so as to adjust the engine output.