1. An object detection system for a host vehicle, said object detection system comprising: an imaging device having a field of view exterior of the host vehicle and operable to capture image data representative of a scene exterior the host vehicle; said imaging device comprising a two-dimensional pixelated imaging array having a plurality of photo-sensing pixels arranged on a semiconductor substrate; an image processor processing captured image data to detect an object present in the field of view of said imaging device; a ranging device; said ranging device tracking the detected object; said ranging device determining distance to the detected object; wherein the detected object is assigned a priority score that represents the potential danger of the detected object to the host vehicle; wherein the priority score is at least in part established responsive to image processing by said image processor of captured image data and to distance determined by said ranging unit; and wherein the detected object is assigned its priority score based on one of (i) the detected object being a vehicle that is closing with the host vehicle and (ii) the detected object being a pedestrian who is present in the field of view of said imaging device.