1. A voice warning system for an automotive vehicle provided with an automatic speed control device, which comprises: (a) a vehicle speed sensor for detecting vehicle speed and for outputting a vehicle speed signal S.sub.p ; (b) a set switch provided for the automatic speed control device for setting the vehicle to travel at a constant speed under control of the automatic speed control device, and for outputting a set switch signal S.sub.s ; (c) a voice synthesizing system responsive to the vehicle speed and set switch signals for reading the speed signal output by said vehicle speed sensor in response to said set switch signal output by said set switch, and for outputting a voice form output message in accordance with the detected vehicle speed signal; (d) an interface for controlling the connections of the vehicle speed signal and the set switch signal to the voice synthesizing system; (e) a volume control for a car-radio in the vehicle, said volume control generating a sound volume signal connected to said interface; (f) said interface including means for generating a voice volume control signal corresponding to said sound volume signal; and (g) an electronic volume control for adjusting the voice volume of the voice form output message in accordance with the voice volume control signal generated by said interface, whereby the vehicle speed is indicated to the driver as a voiced message having a volume level according to the car-radio sound volume as adjusted by driver preference, when said set switch is turned on.