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AP
The checkout line probably slowed considerably when Barack Obama stopped for fruit and cookies in Webster City, Iowa during the 2008 campaign.

At the grocery store, which checkout lane is the fastest? It's a problem we each try to guess the answer to nearly every time we're at the market.

California math teacher Dan Meyer took a look at the problem, and details his conclusions.

There are easily a dozen variables affecting the line speed that have nothing to do with the number of customers in each line or the number of items in their baskets. You could assign some field research here. I spent ninety minutes last week just watching, counting, and timing groceries as they slid across a scanner.

If math is not music to you, you'll hit a few bumps here. He lost me at "The y-intercept is non-zero!" His explanation -- "It should take you zero seconds to purchase zero items but you can't ignore the fixed time cost of the pleasantries ...and the transaction itself." -- makes no sense. Why are you in line to purchase zero items? Don't you just walk out of the store if you can't find anything you want to buy, bypassing the line altogether? How can you purchase zero items?

You're forewarned, but here it is:

What I Would Do With This: Groceries

The bottom line:

# The express lane isn't faster. The manager backed me up on this one. You attract more people holding fewer total items, but as the data 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!

He also notes what we all know: "Check is slower than credit which is slower than cash." But you seldom know in advance who'll whip out a checkbook. Just as you never know who'll need a price check, who'll overspend and start removing items from the belt, who'll fumble for coupons and who'll be unable to find the credit card after all. There really are too many possible variables to yield a system.

Dan doesn't address self-service checkouts, but I find them slower if I have more than a few items: Only rarely do all my items scan on the first try, the extra steps to identify and weigh produce take time, and bagging items myself after scanning takes longer than the bagger who catches items as they're rung up so they're waiting for me in the cart when I've swiped my debit card and punched "OK."

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