So, sometimes you go to a store, and there's just one checkout line, but multiple open registers. Other times, there are lines for each register.

What's up with that?

There's a science to this, believe it or not. Queue management, it's called.

If you think about it, it makes sense, right?

At a bank, one customer might take thirty seconds at the teller window, the next one, five minutes. But, when you're in line, you have no idea how long each person is going to take. So, having one line (queue) but multiple windows helps reduce wait-time.

Most Starbucks have just one line - it's for orders, only, so it moves somewhat rapidly.

Of course, not every customer understands. He or she sees a long line, and gets annoyed or angry. Many will leave the store, even if the one line ends up moving faster than the multiple lines.

Research has shown that consumers routinely perceive the wait to be far longer than it actually is.

In most of the United States, the wait in a grocery store checkout line is negligible — under a minute, Mr. Underhill has found.

Grocery stores are kind of odd. Here, people kind of "self-select". You know, you walk down the row, checking to see how many people are at each register, how much each customer has in his or her basket, and do a mental estimate of how long each line will take. Then, you get in what you think is the "shortest" line - that is, shortest wait.

Most likely, you guessed wrong, right?

CVS is tough - I think the management decided that it was better to have multiple, short lines, even though I think things would move faster if there was just one line. But, customers at CVS want to run in and out, quickly, and if they saw one huge line, they'd walk out, I'm sure.

Why people can't figure it out, I don't know.

The new self-checkout lines at supermarkets definitely confuse people. Sometimes, customers form one line, even though there are multiple registers.

Other times, they expect it to be just like the rest of the supermarket, with multiple lines.

One time, I had a minor confrontation about it.

I was at the front of the self-checkout line, at Shaw's. There was only one line. A guy came up and didn't see the line, so went right to the first register.

"Sorry, there's a line," I said, kindly.

"Only one line?" He said, threateningly. "Hmmm ... interesting."

He scared me. For a second ...

"No," I replied, smiley meekly. "There are two lines ... it's just that they're both behind me."

More: A Long Line for a Shorter Wait at the Supermarket - By Michael Barbaro, The New York Times