Blind Spot Monitor - sensor, visual, audible, warnings - Online

A blind spot monitor is a vehicle-based sensor device that detects other vehicles located to the driver's side and rear. Warnings can be visual or audible. Increased warnings indicate potentially hazardous lane changes.

Volvo was the first to introduce blind spot monitors to production automobiles, their Blind Spot Information System (BLIS) used a camera to visually detect vehicles. Some newer and more costly systems use side radar offering better performance and also warn of fast approaching vehicles entering the blind spot.

Source: wikipedia.org
Photo Credit: benzinsider.com