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; 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; and wherein a beam emitted by said ranging device at least one of (a) tracks the detected object while the detected object moves in the field of view of said imaging device and (b) stays on the detected object while the detected object moves in the field of view of said imaging device.