Which Line is Faster At the Grocery Store?

Dan Meyer recently did an experiment, in which he observed checkout clerks and obtained data from a 6 hour shift at a local supermarket. He found out that the line that is fastest at a grocery store is the one with the least people in it. This is for the most part regardless of number of items each person has, and apparently adding an additional person in line is about equal to adding 17 items to a person's cart.