an audible warning system which reduces the number of land vehicle traffic accidents where there are already traffic control devices installed such as at stop signs , intersection control lights and on vehicles in conjunction with side lights , tail lights and brake lights . the accident reduction results from the monitoring of approaching traffic with a radar - type range sensing system . sound generators , based on parametric - array technology , permit the focusing of sound into waves of a few degrees width and aiming them at a particular vehicle or pedestrian located at a particular range from the sound projector .