The present invention is a monocular computer vision technique. The technique is used to aid road vehicle driving for safety. An camera (e.g., camera or TV camera or camcorder) is installed in a vehicle facing to the road in front of the vehicle to capture a sequence of road/scene images for analyzing the driving situation. The situation includes the distance to a front obstacle or vehicle, the speed of the vehicle, and the left/right location of the vehicle in a road lane.