Why You End Up in the Slowest Supermarket Line

by in News, July 21st, 2014

Why You End up in the Slowest Supermarket LineIs it you, or does it feel like, no matter how hard you try to pick the shortest, fastest-moving line at the grocery store, most of the time you make the complete wrong call and end up crawling along at a snail’s pace, stuck behind someone who needs a last-minute price check on an item or is fumbling around for his or her frequent-shopper card or is simply bent on chit-chatting the afternoon away with the cashier — while customers who come after you and slide blithely into other lines are out of there in record speed?

It’s not just you.

“When you’re selecting among several lines at the grocery store, the odds are not in your favor. Chances are, the other line really is faster,” science writer Adam Mann explains in Wired. “Mathematicians who study the behavior of lines are called queueing theorists, and they’ve got the numbers to prove this.”

There’s an equal chance of a random, unpredictable delay occurring on any of the lines, Mann notes, and if there are, say, three lines to choose from, you have only a one-in-three chance of being in the fastest-moving one — and a two-in-three chance of standing miserably by while the customers in another line move faster than you.

Ushering all customers into a single “serpentine” line — a la Trader Joe’s, airline counters, fast-food restaurants and banks — with the person at the front of the line proceeding to a register as it becomes available, offers a solution. In a three-register situation, Mann says, it’s “about three times faster on average than the more traditional approach.” Plus, if there’s a delay in a transaction at one cashier, everyone waiting behind them for a cashier suffers equally — but not terribly, since other cashiers can keep the flow moving.

“The serpentine line … offers important solace,” Seth Stevenson wrote in Slate in 2012. “You absolutely never have to see someone arrive after you and get served before you.”

So why, if they are more efficient and equitable, don’t all grocery stores use serpentine lines? Customers don’t like them, in part because they look longer, and also because, while it removes the possibility of losing out by picking the slowest line, it also prevents them from winning by picking the fastest one.

“We human beings like to think that we’re in control of our lives and can beat the system if given the chance,” Mann notes. Even if the math and the lady digging in her purse for exact change for half an hour are clearly working against us.

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