It’s common practice in Europe to flash one’s headlights to “communicate” with other drivers. For instance, when a driver is joining a freeway, a single flash of the headlights tells that driver that you are aware of him, and will modify your speed to allow access to the freeway. Likewise, when there’s confusion at a four way stop or other intersection, if one person flashes their lights that gives the other party the okay to go ahead.
Over the years, I’ve watched truck drivers give others the courtesy of a flash of their lights to pull in, move ahead, etc. In Europe, they take it one step further wherein if a driver flashes his lights to tell another to pull ahead, the driver gives one flash of his turn signal in thanks – it’s a nice touch…but I’m reticent to ever flash my lights in The US under any circumstances for fear of upsetting or confusing other drivers.