Imagine any place where you might queue, with two or more possible access points - a couple of cashpoints, a row of train ticket machines, etc. We all know that the best way to approach this is with a single queue that feeds to both/all, but time and time again people fail to act like this. What we need is a subtle social cue that helps reinforce this, without the need for physical barriers or complex explanations.
What we would have is, at the point where the single queue should form, the letter 'Q' marked on the ground. There would then be a series of short lines marked off the 'Q' indicating that this is the point that leads to all of the available machines, as and when they are free. People might ask you, 'are you in the queue?', and you would say 'yes, I am - quite literally! - in the 'Q'!'.
(I came up with this batting ideas back and forth with someone else last night. It was a collective effort to be fair. If you are reading, sorry for squandering our masterpiece like this. Pearls before swine).