Reverse Audible Warning Systems are designed to alert the driver of the presence of any object (including a human) that happens to be behind the vehicle as it is backing up. Most systems utilize sensors that emit high-frequency sound waves that bounce off the object and back to the sensors where they are interpreted to measure the distance between object and vehicle. Warning tones inside the vehicle change pitch or frequency as proximity to the object changes. There are many of these after-market systems, differing in number of components, mounting options, methods of alerting, some also having visual display, etc. All, however, only activate when the vehicle is in reverse gear.