When the driver signals a lane change, the system warns the driver if a car resides in the ‘blind spot’. It may also warn if another vehicle is changing into your lane.
Signals should be both audible and visual, with an audible signal matched to the side where the problem exists. It may also provide a wobble steering effect for a tactile warning.
The system operates only over a pre-set speed limit to avoid false alerts while parking and other lower speed conditions.
Such a system typically requires both radar and cameras on both sides of a vehicle and very sophisticated software to avoid false warnings. The 2013 BMW 5 series offers such an option for $700.
All cars built in October 2014 and later include this feature without extra cost. Due to all the extra sensors, they cannot be retrofitted to older cars.