The express lane isn't faster. The [supermarket] manager backed me up on this one. You attract more people holding fewer total items, but as the data [checkout scanner data from a six-hour shift] shows above, when you add one person to the line, you're adding 48 extra seconds to the line length (that's "tender time" added to "other time") without even considering the items in her cart. Meanwhile, an extra item only costs you an extra 2.8 seconds. Therefore, you'd rather add 17 more items to the line than one extra person! I can't believe I'm dropping exclamation points in an essay on grocery shopping but that's how this stuff makes me feel.
--Dan Meyer, dy/dan, on fixed versus marginal costs of supermarket checkout. HT: Lifehacker