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

such a flash.

The signal is primarily used

between truck drivers. However, you may also encounter it as a car driver. This happens, for example, when you are traveling with a caravan or trailer. With just a passenger car, you can usually clearly see when you have passed a truck. With a caravan behind the car, that is much more difficult. Your vehicle combination is suddenly several meters longer.

A truck driver may therefore also briefly signal to you with his headlights. By doing so, he lets you know that not only your car, but also your caravan, has passed