Reb, supermarkets in my area have changed their signs to say "About 10 items or less." which seems to indicate 11 is ok.
Also, your other logic is flawed. If the sign says 10 and 11 or 12 doesn't bother me, that doesn't mean that 13 can't bother me.
I have found there's a stronger correlation to number of *transactions*, and not number of *items per transaction* to how long one waits in line. 10 people with an average of 4 items in the Express lane or 1 person with 50 in the "slow" lane? Go with "Alice" (from the Brady Bunch show): it doesn't take that long to swipe items; it takes most people effing *forever* to pay. Besides, "Alice" is organized and the 10 dopes in the Express lane are just back for stuff they forgot the other day or they're stoned and buying munchies. Get my drift? I'm not sure where the equilibrium point is in the equation to tip it one oway or the other, but you'll get a feel for it if you practice. Just don't be a slave to your item count.