I've wondered as well. I worked as a cashier in a grocery store when I was in high school. At our store we had two "main" express lines, one for twelve items

I've wondered as well. I worked as a cashier in a grocery store when I was in high school. At our store we had two "main" express lines, one for twelve items or fewer, one for eight. Except for very quiet times, when there was only one cashier on duty, the twelve item line was open and the eight item line wasn't. (Any of the other lines could also be turned into express lines by setting a switch at the register, but they were less clearly marked, so lots of customers didn't notice them. They were only rarely activated.) Your intuitions square with mine: payment was usually the biggest time sink, especially with checks and cards. Still, there were plenty of sources of variation: some items were hard to scan quickly, some customers had more produce that needed to be weighed, sometimes an item wasn't in the database or there was a dispute about its price and it needed to be checked, some lines had baggers--excuse me, service clerks--and some didn't. And, of course, some cashiers were just faster than others. Interestingly, there was no clear pattern here. Experience (having produce codes memorized, knowing how to scan tricky items or even having their UPC codes memorized, and being good at counting cash quickly and accurately) played a role. Mostly, however, people seemed to be intrinsically motivated to do their work well, or else they weren't. We high school kids weren't collectively faster or slower than the "old people." Some customers were faster than others too: cash payment was definitely faster than card, and both were much faster than check. (This was 15 odd years ago; card is likely faster than it used to be, but likely still slower than cash.) Some customers insisted on paying with exact change, and many of them would take their sweet time doing so. Also, some customers would come with lots of coupons, which would take time to scan. My hunch at the time was that we'd have gotten better throughput by keeping the eight item line open, cash-only, no coupons. I think I was probably right, given the constraints: we were going to have an express line, regardless of whether it was really more efficient or not. Might as well optimize it. Now, I think that several feeder lines would be more efficient. Maybe just two (express and regular), maybe three or four (although I distrust people's honesty as well as their ability to count). http://blog.mrmeyer.com/?p=4646 http://blog.mrmeyer.com/?p=4646 has shown up on HN at least twice, and it's worth reading.