The sensors that use ultrasonic sound waves will typically have a range of around 15 feet. At that range, they will pick up the approach of the vehicle and turn green. They will turn yellow when the vehicle is about 8 foot from the stopping point and red when the vehicle is perfectly placed. Parking aids that rely on contact will, of course, have a far shorter range. Since they work by contact with the vehicle, their range is in fact technically 0 inches. On the other hand, even physical, low tech parking aids are still designed to provide a visual reference point to help with parking. Their range is therefore limited only by your eyesight!