Last night as I was driving on the interstate I had a car from behind me start turning their lights on and off. Other times I've been driving at night I've had cars flash brights on and off right behind me. In the U.S. where I come from this would be a very aggressive thing to do on the road. I might think some nasty thoughts about what they want or why they are doing it and I (more likely) might get a bit scared thinking about their intentions. But here, it is a common courtesy to let the person ahead of you know that you are going to pass them. It's a road courtesy to prevent accidents. I am hoping that I don't adapt too much so that it overflows into my stateside driving skills!