A lane departure warning system watches the road ahead and sounds an alert when the car drifts over a lane marking without a turn signal. Cameras mounted near the rearview mirror read painted lines. Some systems add radar or lidar for redundancy.

The feature entered production on commercial trucks in Europe in the early 2000s, then spread to passenger cars. It is now standard on many models and required by regulation in the European Union for new vehicles since 2022. The related lane keeping assist goes further and nudges the steering back toward center.

What triggers a warning

• Crossing a solid or dashed lane line without signaling.
• Drifting during driver fatigue or distraction.
• Poor lane markings can cause false alerts or silence.
• Heavy rain, snow, or direct sun can blind the camera.

Drivers often disable the system because the beeps annoy them. Automakers have responded with steering wheel vibrations, seat pulses, and gentler chimes. The technology works best on clear highways, not on winding back roads or construction zones with temporary markings.