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.

Any delay in the multiple-line system will stop the line completely, while a delay in a single line might just delay one shopper. Although the single-line method may be faster, Mr. Hammack says customers generally prefer to "jockey for position" in separate lines.