My current charabanc has its fog lights in the main lamp cluster along with dipped and main beam. If you drive with your dipped lights on (always do) and then alternately flash fogs and mains as you come up behind someone, they almost always move over, it looks too much like the flashing pattern on unmarked police cars.
Saturday night, I spent a couple of miles stuck behind a middle-lane hog, driving past an empty inside lane and unable to get out into the melee of the outer overtaking lane, despite flashing lights etc. etc. When the outer lane finally cleared, I was able to pull out and I stayed alongside the hog until we came to another, slower middle-lane hog (not totally sensible, but to give him an idea of the holding-up effect he was causing). What did he do? Pull into lane one and pass on the inside, only to pull back into the middle! I was so stunned I hadn't managed to complete the overtake and pull back in in time to balk him.