At 3 in the afternoon, the servers and cashiers have plenty of spare time to bus all of the tables.

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In a research project conducted during last year's holiday season and posted on YouTube, Bill Hammack, a professor of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign, concluded that a single-file line leading to three cashiers is about three times faster than having a separate line for each cashier.

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It's now tougher for cashiers operating in tandem with friends from the outside to casually omit the scanning of some items at the counter.

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