What is blind spot warning (BLIS)?

Blind-spot warning (or BLW, or BLIS) is a driver-assistance system fitted to some cars that shows a warning light – either in the door mirror or on the inside of the door itself – when another car is in your blind spot. Blind-spot monitoring systems use sensors on the side of the car to monitor the movements of other cars in your blind spots. Some systems will beep at you if you try to indicate to change lanes when there's a car in your blind spot, while some will vibrate the steering wheel as a warning. It's an especially useful system on larger cars with poor rearward visibility.