1. A vehicular hazard warning system comprising: an indicator on the vehicle and energizable to yield a warning signal; means for detecting the onset of a hazardous condition of said vehicle and developing a control signal having a variable parameter the amount of which is proportional to the degree of hazard represented by said condition; means responsive to said parameter for energizing said indicator in a manner which quantitatively has a magnitude that is proportional to said amount; said detecting means including an accelerometer that develops said control signal and effects variation of said parameter in proportion to the degree of deceleration of said vehicle, said accelerometer including a first inertial rotor driven in rotation at a speed representative of the velocity of said vehicle; a unidirectional clutch; a second inertial rotor coupled by said clutch to said first rotor for rotation of said rotors in unison upon acceleration of said vehicle but being uncoupled from said first rotor upon deceleration of said vehicle and thereby enabling rotation of said second rotor relative to said first rotor; and means for effecting variation of said parameter in proportion to the speed of relative rotation of said rotors.