Dan Meyer spent 90 minutes watching, counting, and timing groceries as they slid across a scanner — and he found out that each extra item costs you less than three seconds, but each extra person in front of you costs 48 extra seconds:
I don’t find it at all surprising that the y-intercept is non-zero, and that one more person adds as much time as 17 more items — or that the express lane isn’t faster.
(Hat tip to Tyler Cowen.)