idiots in the overtaking lane
Or idiots that forget the line at the bottom of the slip road is a giveway line (amazing there is usually no queue to get on the m25, just a huge queue on the m25 as people force their way on regardless....
But anyway, the idiots who wont move over already wont for lights and horn so what makes you think they might for a polite message? I have found a much bigger, much older, already battered looking car is a most efficient way of moving people... an occasional bump might be needed but when they see the state of the car nudging them they usually give way