Is the express lane in the grocery store always the fastest lane?
That’s a great question and its the subject of a “Dear Mona” column over on FiveThirtyEight (Dear Mona, Which Is The Fastest Check-Out Lane At The Grocery Store?, Oct 16). Mona attacks this question by heading into the queuing theory weeds.
I couldn’t find much research on express lanes specifically, but one paper from Amsterdam found the reduction in wait times for express-lane customers didn’t offset the overall increase in wait times for everyone. Maybe life would be easier if the supermarket didn’t have an express lane — or, better yet, if it got rid of multiple lines altogether and had all customers join a single infinitely sprawling line where there were no winners and losers. That might sound nightmarish, but the math actually suggests it would be anything but.
That math comes from queuing theory, a subject of study that’s been around ever since Danish mathematician Agner Krarup Erlang discovered a method for managing telephone traffic in 1909. To answer your question, I’ve had to take a crash course in (more modern) queuing theory, including examining formulae that calculate how average wait times at the grocery store vary depending on the type of line you join.
I should state upfront Mona on the whole acquits herself quite well on this. But there are a couple of points worth mentioning. First, there in fact supermarkets that run with a single queue, like this Hannaford’s in West Lebanon, NH.

As you can see, that singe serpentine queue ends up chewing up a lot of space at the front of the store. That’s a lot of real estate to give up when you only have two people in line. As we have written about before, that is only one of the complications of having a single queue in a grocery setting.
But let’s suppose for the moment that we can get a single queue to work. Is that in fact the best way to run a supermarket’s checkout? (more…)
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