After overtaking, the truck behind you flashes briefly once: this is what the driver wants to tell you.

vehicle combination has been passed.

This can be particularly relevant when trucks are traveling at approximately the same speed. The speed difference during an overtaking maneuver may sometimes be only a few kilometers per hour, meaning that the vehicles can travel alongside each other for quite some time.

The truck driver being overtaken often has a clear view of the rear of the other vehicle. Once the entire vehicle combination has passed, the driver may briefly signal with the headlights. This can indicate that the overtaking vehicle appears to have