A motor vehicle speed control system for attenuating sudden unintended acceleration and maintaining driver control of a motor vehicle has a brake switch connected in series with a vacuum switch and a relay. A foot pedal when depressed actuates the brake switch to provide electric power to the vacuum switch coupled to the intake manifold of the motor vehicle. Vacuum pressure in the intake manifold actuates the vacuum switch to close the electric circuit to the relay. Relay interposed in the electric ignition or fuel injector circuits is open when both brake and vacuum switch are closed thereby interrupting these circuits and reducing the power or terminating the operation of the motor vehicle's engine.