We tried to have one line at our self-checkout area (two rows of three scanners each) for the longest time for this exact purpose. We even had a queue set up with ropes and everything. People just walked around the ropes and insisted on forming a line in front of each row. Then, when they got stuck behind three people who were technologically challenged, they would try to hop over to the other row and grab the next register, cutting off someone who had been waiting in that line. Eventually, we gave up on the single queue and reconciled ourselves to breaking up the screaming matches that resulted from two lines.
Apparently, some people really, really do not like the idea (or more likely the visual appearance) of a long line. They would rather form several shorter lines, even if the single line is far more sensible.
In my experience as a cashier, about nine out of ten check-writers are senior citizens who’ve been paying for their groceries with checks since the Crimean War and ain’t got no nevermind for no electification nohow, consarnit.
Except, any store with “ungodly” lines which is considering the various approaches mentioned in the thread is probably already losing customers by the delay in all its lines. So it’s not clear that losing a slow or illiterate customer, the kind that holds up the others, is a net loss.
I think there should just be time limit lines. Once the cashier says the total is X, you got Y seconds to complete the transaction. If you exceed Y your shit drops down a trap door and you get flung out of the store by a catapult.