In the USA, I at some point picked up flashing headlights as "thanks", I don't know how common it is, I don't see it that often (anymore?), but USA drivers may be unlikely to give thanks on the road. :|
(Flashing headlights is also "i see you trying to merge in front of me and i'll give you space." Which I also occasionally do but don't see that often).
(Flashing headlights is also "i see you trying to merge in front of me and i'll give you space." Which I also occasionally do but don't see that often).