A man holds a shopping basket with groceries while
waiting in line on May 9, 2014, at a Carrefour market shopping center in Paris.
JOEL SAGET / AFP / GETTY IMAGES
Dear Mona,
Is the express lane in the
grocery store always the fastest lane?
Barry, 44, New York
Dear Barry,
We’ve all been there. While grasping a stack of toilet
paper and a carton of milk in the grocery store’s express lane, you glance to
your right and see that other customers with cart-loads of produce are
seamlessly drifting forward in their appealingly shorter lines. You start to
contemplate whether you should stay put, switch lanes or just head home
empty-handed to face dry cereal and your wet bum.
I couldn’t find much research on
express lanes specifically, but one paper from
Amsterdam found the reduction in wait times for express-lane customers didn’t
offset the overall increase in wait times for everyone. Maybe life would be
easier if the supermarket didn’t have an express lane — or, better yet, if it
got rid of multiple lines altogether and had all customers join a single
infinitely sprawling line where there were no winners and losers. That might
sound nightmarish, but the math actually suggests it would be anything but.
That math comes from queuing theory, a subject of
study that’s been around ever since Danish mathematician Agner Krarup Erlang
discovered a method for managing telephone traffic in 1909. To answer your
question, I’ve had to take a crash course in (more modern) queuing theory,
including examining formulae that calculate how average wait times at the
grocery store vary depending on the type of line you join.
There are several types of lines
we could consider — textbooksclassify queues according to how many servers
there are, whether a customer is only served once (like at the dentist) or in
multiple phases (like at a drive-through), as well as whether she gets to
choose which line she joins. Since you’re interested in your grocery store,
where there are almost certainly several servers and one phase of service, I’m
going to focus on that last distinction: Are single lines better than multiple
lines?
Queuing Typology from Supplement C, Waiting Line
Models.
HERBERT BLAKE, JR.
But to make that comparison, I’d need to know things
like the number of cashiers at your grocery store, the number of items
customers have in their carts and the amount of time it takes to serve each of
them. There isn’t any national data we can plug in to the formula to give us
answers — the numbers vary considerably by store and even within a store, and
will change on a weekly, daily and hourly basis. One alternative, then, is to
run a simulation — pretend that 10,000 hypothetical people are waiting in your
hypothetical store and see how their wait times vary.
That’s what Wes
Stevenson, a professional data analyst, did out of sheer curiosity.
To test the difference between single and multiple lines, Stevenson quantified
a few assumptions (10 cashiers, an average wait time of 3 minutes) and put them
into his model to see what it would spit out.
He found that the wait time in
single lines is more predictable (you can see that in the chart below, where
there’s a narrower spread of outcomes). More importantly, though, Stevenson
found that a single line is more likely to mean a shorter wait (also visible
below, where the single-line chart clusters farther left than the multiple-line
one, meaning that more of those 10,000 simulations produced a shorter wait
time). In that respect, Stevenson’s finding concurred with well established queueing theory: One line is better
than many.
Queueing analysis by Wes Stevenson of
statistical-research.com.
STATISTICAL
RESEARCH
“But wait a minute!” I (sort of)
hear you cry, Barry. “If single lines reduce wait times by so much, why do
grocery stores across the country have us queueing in separate lines for each
cashier? And presumably the 830,750 Americans who work as cashiers in grocery
stores would also benefit from having to deal with fewer pissed-off customers?”
One reason is that models like
these overestimate the difference between single and multiple lines because
they don’t take into account some human behaviors. Maybe you know your grocery
store pretty well, Barry. Maybe you know who the fastest cashier is and which
register has the drawers that always get stuck; maybe you can even spot that
customer that always spends an age rummaging
around for his wallet — and you pick your lane accordingly. Aside from careful
queue selection, you might also switch lanes (known as “jockeying” in the
queueing theory textbooks) or simply ditch your items and leave (“reneging”).
Those behaviors, from you and other customers, reduce the average wait time in
a multiple-line queueing system and bring it a little closer to a single-line system.
To be fair though, if the models underestimate human
intelligence, they also fail to factor in just how mistaken we can be.
For one thing, our perceptions don’t always match up
with reality, and that matters to the businesses that want us to be their
customers. The longer we stand in line, the more the gap between perceived and
actual wait time grows. By the time we’ve been in line for 5 minutes, we think
we’ve been waiting for 10, according to Paco Underhill, who measured shoppers’
over-estimations for his book “Why We Buy: The Science Of Shopping.”
Unsurprisingly, longer perceived
wait times means lower satisfaction. Or, to be more formulaic, perceptions feed
into a simple equation written by David H. Maister from the Harvard Business
School: S = P – E, where S = satisfaction, P = perceptions and
E = expectations. One way then, Barry, to feel more satisfied with your grocery
store experience (and your job, and your love life and just about anything) is
to simply lower your expectations. I guess that probably doesn’t sound too
appealing.
It’s not just perceived waiting
times that show how we as customers don’t have perfect judgment. Imagine your
reaction if your local grocery store did switch to a
single line. You walk in after a long day at work, and rather than seeing 10
neat lanes with four or five customers in each, you’re now met with a winding
line of dozens of bodies. It’s probably harder to gauge the rate at which the
queue is moving ahead than its simple length. Maybe you’ll turn straight back
the way you came. That U-turn is known as “balking” in queueing theory and,
from a business perspective, it’s a store’s worst nightmare.
So, rather than simply shoving us
all into one line, supermarkets are exploring a range of alternatives to reduce
both our actual andperceived wait times. I spoke to
Perry Kuklin, director of marketing at the queue-management consultancy Lavi Industries Inc.,
about the sorts of methods the firm suggests to its clients, who range from
pharmacies to personnel services at U.S. army and naval bases.
Kuklin outlined three main strategies that Lavi’s
clients are using to improve the queuing experience. First, customers waiting
in line can have their items scanned by roaming tellers with hand-held
machines, to reduce their service time once they finally reach the cashier.
Second, customers can register their place in line, go away, and come back once
it’s their turn. There’s nothing new about this latter strategy, really, it’s
just that rather than grabbing a paper ticket from the deli counter and waiting
for your number to be called, you might receive a text message saying you’re up
next.
But it’s a third strategy that
has really stood the test of time: distraction. “You can try to entertain
customers with videos,” Kuklin said, “and in-line merchandising helps to keep
them busy and distracted.” There are more cunning ways, too. After airline
passengers wouldn’t stop complaining about the time they
spent at baggage claim (even when more staff were added and wait times fell) a
Houston airport simply moved the arrival gates so that passengers spent more of
their “wait” time walking to the baggage claim. The complaints all but disappeared.
All of this shows that we who stand there desperate,
tired and irritated are not perfect. But we’re also not perfectly imperfect.
So, until your grocery store switches to a single-line system, my advice to you
is: Be observant, and (if you don’t want to examine chewing gum and candy bars
for a distraction while you’re waiting in line) take a book.
Hope the numbers help,
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