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

means the same thing.

If you see a truck flashing behind you, pay close attention to the moment it happens. Does the flash occur exactly after you have 

overtaken the truck? Then there is a good chance the driver is trying to help you. If the signal occurs at a different time, something else

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might be going on. Perhaps the driver wants to warn of danger. It could also mean that another traffic situation requires his attention.

That is why you can never rely blindly on a single light signal. View it primarily as additional