What feature warns the driver if a vehicle is in the blind spot?

The feature that warns a driver when a vehicle is in the blind spot is a blind spot monitor.

A blind spot monitor uses sensors—often short-range radar, cameras, or both—to detect vehicles alongside or slightly behind the car where the driver may not see them through the mirrors. The system usually gives a visual warning in the side mirror or instrument display; some vehicles add an audible alert, steering-wheel vibration, or a warning when the turn signal is activated.

The technology became closely associated with Volvo’s BLIS, or Blind Spot Information System. Volvo showed the feature on its 2001 SCC concept car and introduced it in production on the 2003 XC90. The Ford GT90 concept is also often cited as an early vehicle fitted with a modern blind-spot monitoring system.

A blind spot monitor is different from rear cross-traffic alert, which warns about traffic approaching from the sides while reversing. Forward collision warning watches the road ahead, and lane departure warning detects unintended movement out of a lane. Blind-spot technology is an aid, not a substitute for mirrors, turn signals, and a shoulder check.