1. An imaging system for a vehicle, comprising: an imager configured to image a scene external and forward of the vehicle and to generate image frame data corresponding to each frame of a series of acquired image frames; and a processor configured to receive and analyze the image frame data to: detect objects in the scene and a dominant motion of the scene, and determine relative motion of the detected objects relative to the dominant motion of the scene, wherein the analysis of the image frame data performed by said processor includes categorizing the detected objects using the relative motion of the detected objects, wherein said processor categorizes objects detected in the image frame data using one or more of a detected brightness, size, position of center of mass, age, and color of the objects in addition to the relative motion of those objects, and wherein, when categorizing objects, said processor increases the probability that an object is a vehicle if the object has a high relative motion.