I agree with the other posters here, this is a really bad article. Even if the effect is true, it doesn't matter because nobody cares if you take 20 seconds or 30 seconds to check out. I care about the fact that when cashiers had their own lines, I had to stand for 5 minutes behind someone who tried to use a points-based rewards card, didn't have quite enough on it, then the cashier went through something complicated so she could deduce what he did have, document and sign it, and have him pay the tiny cash difference between his sum and what he had on his card. It literally took 5 minutes, while I was trying to buy like milk and bananas or something. Of course, I thought about the well-known research on single queue. When I saw other lines moving, I wished I had been in one.

plus, this appears to be the result of an experimental setup rather than real-world facts. It's trivial to get real-world data from some place that implemented the difference, since all receipts are time-stamped. You just have to count the number of receipts in single-line, vs everyone-with-their-own-line, and see which is bigger. (You can also see how average total or number of items differs between setups, or normalize for any changes, such as only considering the time taken to ring up literally the exact same order.)

this isn't data rocket science. any store which went single-line should have this data.