Single flash of hazards to thank someone is common in Brussels traffic, there are a lot of very selfish drivers on the road who never let anyone merge in. Haven't seen it outside of here, maybe because people are usually a bit more polite or zipper merging works as it should.

Common in all of Belgium to put hazards on to warn people behind you that there is a traffic jam up ahead. I really like that passing the baton system.

Also single flash of high beams to notify someone you've seen them and they can cross or pass where they'd otherwise not have right of way. While you should always be as predictable as possible, sometimes doing it wrong resolves the situation faster for everyone.

I also always raise my hand as a thanks when someone stops to let me use a crossing (I have the right of way as a pedestrian but I still feel like sending people a thank you for observing the rules correctly). Did that when walking in Brussels this morning out of sheer habit, realized that is probably actually pretty unusual for a big city. My small town habits are betraying me.