Tunnel detection changes the lighting from daytime running lights to dipped beam when the car is driven into a tunnel.
The tunnel detection function is available in cars with rain sensor*. The sensor detects the entrance to a tunnel and resets the lighting from daytime running lights to dipped beam. Approx. 20 seconds after the car has left the tunnel, the lighting returns to daytime running lights. If the car is driven into another tunnel within this time period then dipped beam is kept switched on. This avoids repeated changes to the car's lighting.
Note that the headlamp control's knob must remain in position for tunnel detection to work.