+1 on AlmostTrick. I think what happens is that your brain hears "left" and takes it not as an advisement, but a command. And of course, telling left from right is a problem for many, many people. If you had time, and if people wouldn't find it automatically insulting, I'd love to be able to say "coming up behind you, please keep your line." But really, a pleasant warning well in advance seems to be the best way. I'm a big fan of the "ding-ding!" kind of bell (why do the single-ding ones just seem less nice?).