single collision condition in which a single object i e one of legs of a pedestrian is being hit by the vehicle which is detected at the time t 2 if two of the distances x 1 x 2 and x 3 become nearly equal to each other it also means that the object vehicle collision condition has been returned from the multi collision condition to the single collision condition moreover if the distances x 1 and x 2 are identical with the distance xm 2 or the distances x 1 and x 3 are identical with the distance xm 1 it also means that the object vehicle collision condition has been returned from the multi collision condition to the single collision condition further between the times t 0 and t 3 the distance x 1 always changes in each of the events at the times t 1 and t 2 thus if after a change in distance x 1 is detected a change in either of the distances x 2 and x 3 exceeds a given value it may be concluded that the time t 2 has been reached furthermore the distance x 1 changes by a given amount when the multi collision condition is encountered at the time t 1 and also changes by a given amount in the same direction i e toward the center of the vehicle in the example of fig1 upon disappearance of the multi collision condition at the time t 2 a quick determination of whether a pedestrian is hit by the vehicle or not may therefore be made by starting the.