This evening on the motorway I checked my mirror and I could not believe how close the car behind was. So I turned on my hazard lights, and he immediately changed lanes. Sorted. But I do wonder if this could have issues, e.g. frightening a tail pipe sniffer into a risky manoeuvre and causing a crash. Usually I'll try and ignore them, and make sure I have a lot of space in front, but the closeness really shocked me. Sometimes I change lane, let them go past, if possible. I wouldn't try the brake light trick - dabbing the brake - as that could be dangerous, or provoke a dangerous retaliation from the nut job behind.