The following invention is a passive collision avoidance alarm system. An optical sensor (which may optionally be visable, infrared or ultraviolet) is disposed to provide a continuous raster scan of the scene within a wide angle of the direction of travel of the vehicle. This sensor output is converted into digital data and stored. A computer system compares consecutive scenes to detect identifiable objects. For such identifiable objects the computer calculates the centroid of the object and its angle, and a measure of the size or extent of the object. Detection of an object having a constant angle and a increasing measure of extent causes an alarm to be triggered.