A blind spot monitor is sensor- or radar-based technology that warns drivers who are changing lanes if a vehicle is at their side, but invisible to them.
A blind spot is the precarious position where the drivers' side view mirrors cannot perceive lateral traffic. Some motorists look over their shoulders to compensate for this loss of visibility, and this measure is mostly effective. However, more than 300 deaths per year relating to blind spot accidents indicate that not all efforts work.