Abstract: The aim of the invention is to generate a timely triggering signal for an effective unfolding of an airbag and, at the same time, to take the relative velocity of the collision opponent into account as a triggering criterion. To this end, the invention provides that two interspaced sensors, which emit millimeter waves, are integrated in each vehicle body door. The measuring signals of said sensors are generated by means of a multi-frequency modulation method. The invention also provides that during a signal processing operation using radar technology, the obtained measuring signals are turned into velocity values (Vn) and distance values (R) according to individual echo centers of the collision object. In addition, the transversal velocity (Vq) with regard to the door surface is detected from said values in a series-connected situation analysis module. A classification of the collision object results from the measured values and a criterion is obtained from a subtraction of both sensor measurement outcomes.