Joost Vles writes about a subject that I have long had an interest in and that is queuing theory. I have noticed that airports and banks tend to favor the single line system but grocery stores go for the separate queues. He points out what should be obvious but that some people do not seem to realize, and that is that a single long queue where the people break at the head of it to go to the next available server is better than each server having their own queue.
Seeing a line snake back and forth across the width of a store three times can be deceiving as to how long you may actually have to wait. In what may appear to be a very long line, the service rate can be so good that the line moves very quickly.
Imagine a situation where you have many shorter lines, each being served by its own cashier. Call it the grocery store model, or the single-server model, more officially. You can get out of there quickly only if you correctly guess which line will move the quickest. And if you’re anything like me, you’re bound to bet on the wrong line.
But a single, longer line, being served by multiple employees – think banking, the motor vehicle department or airport security – is actually faster for everyone, even though it looks much longer than what you’re used to seeing in other systems.
The main reason is that if there’s a price check, a return or some other very slow customer, that delay affects only that cashier directly dealing with the situation. The rest of the line continues to move along. The delay at one cashier gets distributed across the entire system in the multiple-server model, instead of completely stalling out just that one line, as in the single-server model we see in the grocery stores.
Vles says that designing the most appropriate queue is based on Little’s Law that states that “over time, the number of customers in a system is equal to their rate of arrival multiplied by the average time they spend in that system” and that some systems (like mechanized car washes) have fixed times of service while others like the post office have varying times.
The place you are most likely to encounter the single-server model is the grocery store, perhaps because the layout of the store is not conducive to having a single long line, even if that is more efficient. But another factor may be that people get annoyed by seeing long lines because they do not appreciate that it is actually more efficient. As a result, when one goes to a grocery checkout, one has to guess which line is likely to be the quickest. It really does not matter that much since the difference in times between different lines is not that great but there is something annoying about joining a line that seems the shortest and then getting held up because of a customer ahead of you while the neighboring longer line zips along quickly. It feels like losing a bet.
Over time, I choose grocery store lines using a rule of thumb that is blatantly ageist and sexist. Other factors being roughly equal, I tend to join lines that have only men, the younger the better. This is because men tend to load their stuff on the conveyor belt and pay whatever they are told, no questions asked. They never seem to notice if the price charged for an item is different from what it was displayed on the shelf or was advertised in a store flyer and ask for a price check. When it comes to payment in cash, they never try to pay the exact amount by digging through their pockets to find the exact change but simply give large bills and not look too carefully if they get the right amount of change back. They never write checks. They never have coupons. I doubt that they would notice even if an item they purchased was not put into their shopping bag. Women, especially older women, tend to be much more observant and careful shoppers. As a result, lines of men move faster than lines of women.
I have of course not done any research on this subject and am generalizing based on my own experience, never a wise thing. For example, all the male characteristics I have listed above are strongly based on my own behavior. Sweeping generalizations are course never true in every case and my personal impressions may be too restrictive or only representative of my older generation.
But since grocery shopping is something that everyone has to do, I am curious as to what readers think, whether these generalizations have any resemblance at all to their own experience.