Why the Line Next to You is Not Moving Faster

You know when you’re in a supermarket or department store and you’re deciding which line to choose? It’s always the same sad story. You finally pick, then you switch, switch back, and then wish you’d stayed in the other line. Well, Bill Hammack, aka “the Engineer Guy,” advises shoppers to leave it up to the fates. The odds are in no one’s favor when there’s a separate line for each cash register.

Unified queues with a single line leading to different cash registers are a more efficient method for getting people cashed out quick. So pay attention store owners and managers – this one’s for you. Follow the lead from stores like Marshalls and T.J. Maxx because according to Hammack’s theory, they’re doing it the right way.

Back in December of 2010, Lifehacker released a brief article and an accompanying video addressing this line-next-to-you phenomenon.

Lifehacker posted this video just before Christmas in 2010, and while the video itself is a couple years old, its relevance still holds today, and will hold for a long time to come. At least until stores across the world begin cashing customers out with a unified queue.

In the video, The Engineer Guy explains his argument by juxtaposing queueing theory with early telephone technologies.

The original question asked by Agner Erlang from Copenhagen Telephone Co. in the early 1800s, was how many trunk or main telephone lines are needed to adequately service a town? There were two extremes the phone companies were considering:

1. One line for the entire town – the problem here would be delays due to blocked calls.
2. One line for each phone – this was thought to be too expensive and wasteful because people don’t make calls all at once.

Their solution: Figuring out the average number of calls in an hour and the average call duration. Then they found how many trunk lines they’d need in order to make sure that only 1% of the people would experience a blocked call.

How this relates to Queuing: A shopper approaching a cash register is like a phone call arriving, and an open cashier is like an available trunk line.

Most stores just figure the average amount of customers arriving in a typical hour and then assign a certain number of cashiers to be sure everyone is served semi-quickly. However, as Erlang showed with the phones, people don’t make calls at determined times and rates. They make calls in bunches, and one cannot be sure when that will occur. The same applies for stores. People arrive to the registers in bunches, and no one can be sure when exactly that will occur.

The Engineer Guy figured out that unified queues (or single lines) that feed to different registers, are three times faster than individual lines for each cashier. Why? Well, with a single line for each cashier, any sort of delay – like an elderly woman slowly probing her purse for coupons – will slow down the movement of that particular line and cause a gridlock.

When you have a unified queue, the person next in line will not be subject to that delay, because they will simply move to the next available cashier.

Think about the queueing setup at banks. There are always unified queues at banks, because each customer has different needs, and accordingly, will take different amounts of time with the teller. Thus, one line serves best so that no customer is subject to longer wait times.

The Engineer Guy argues most stores don’t do this because it bothers the customers psychologically. He argues that people prefer the line jockeying game that we all know too well.

He goes on to explain why it seems that the other line is always moving faster through an explanation of the randomness of delay occurrences. For a more detailed explanation, watch the video, because this aspect is difficult to explain without an adjoining diagram.

And for shoppers – remember, when you’re at a store with individual check out lines, leave it up to the odds, because it’s in fate’s hands.