# What is the benefit of 2 drive thru lanes at a fast food restaurant?

The following photo shows the drive thru lane at a fast food restaurant. The lane splits in two to allow 2 drivers to place their order at the same time.

The following photo shows an overhead view of the restaurant:

As you can see, the two lanes converge back into one lane before the payment window. Therefore the payment and receipt of food is still done serially, which would seem to negate any benefit from orders being placed in parallel.

In addition, there are downsides to having more than one drive thru lane. A driver in one lane might not pay any attention to the other lane, which would increase the risk of an accident. Also, due to orders being placed simultaneously, the person at the payment window repeats the order to each driver to confirm that the orders/drivers are in sync. On more than one occasion I have had the wrong order repeated to me.

So what is the benefit of having two drive thru lanes?

• Your question, "What is the benefit of 2 drive thru lanes at a fast food restaurant?" logically lead to the question, "What is the benefit of more than one queue inside a fast food restaurant? The benefits are the same. Feb 17 '17 at 1:43
• I'm sorry that you've had such a bad experience: "On more than one occasion I have had the wrong order repeated to me." I've never had that happen. (And I've visited drive-thru lanes quite a lot.) Actually, I've seen local staff handle this well. For instance, if two people finish ordering at the same time, one staff member would delay a bit longer before responding with the order-recap/total, thereby controlling which car advanced first, so that the natural but avoidable race would be far less likely to occur. Feb 17 '17 at 3:45
• Placing the order is the part that takes the most time, as there are variables. The payment and collection phase aren't variable - you're told a price, you hand over the currency and you're given what you ordered. Feb 17 '17 at 9:54
• @Makyen, a pretty obvious explanation is that ordering takes longer than paying. The longest process is parallelized. Feb 17 '17 at 17:51
• @Makyen, whoosh. :/ I understand now. Feb 17 '17 at 22:32

I am in the fast food business having owned five units for over 30 years but don't have a unit with two lanes.

The reason for two lanes is a matter of timing from McDonald's research into this that I read years ago. (I am not with McDonalds.) It all comes down to timing and shaving 10 or 20 seconds off the time you wait in line is worth having. The slowest part in this queue is ordering. Not everyone is slow but the average is. And if one line gets bogged down by one customer, half of the other customers are in another line and continue to flow, hopefully.

In addition, it makes the line look shorter. So it's partially psychological but combined with increase feedthrough it makes the customer experience better. It's better to be third in line of two lines than sixth in line of one line.

Of course, sometimes it's a matter of parking space and two lines prevents cars from spilling out into the entrance from the street.

This is how I look at things. First customer comes in and gets waited on right away. Everything is great. Second customer comes in but he's happy cause he's next! But the third customer in thinks, "Aw, man, how long is this going to take?"

EDIT: The queuing theory, and all similar ideas, are very much applied to these things. Fast food operations are small factories that produce packages of meals no matter what anyone wants you to believe. Just like a plant, timing of operations is studied from before the restaurant opens to when a customer walks in the door to the time it closes.

This flow is just as important as anything else. I know how long it should take for a single customer at the head of the line to get through to checkout. I know how long it should take for the customer at the end of the line, too. If it's taking longer than those max times, my people make adjustments like taking problem customers out of that flow.

I've always said that, in my restaurants, if something goes seriously wrong at any point in our morning setup, it takes us a full 24 hours to recover. The customer won't always notice but we do.

• +1 Because this is the only answer (till now) mentioning that maybe the true reason is not UX, or the number of customers served, but just optimal use of the available space. Feb 17 '17 at 8:09
• I voted for Robb since experience wins over theory. Never the less I have some comments: 1. there are TWO UI aspects: 1. the people in the queue and 2. the people serving the queue. This would increase the customers satisfaction, possibly increase the workers satisfaction (the happier the customer is the nicer they are to deal with), but may make the work a bit harder for the worker. Feb 17 '17 at 9:24
• @iheggie I'd say that the experience confirms the theory. Queueing theory is a whole area of mathematics and it tells you all kinds of useful things. For example, it's better to have a single queue shared by all the servers than it is to have a separate queue for each server. Feb 17 '17 at 10:26
• @david - Queuing theory doesn't state that a single queue shared by all servers is always optimum! Instead it gives you the intellectual model and maths to determine which is best. Where the time to take the order exceeds the time to take the money or to serve the food then overall efficiency will increase with multiple queues at that point. It also allows rudimentary prioritisation by time taken to order +incentive to order quickly to get ahead. Your web request doesn't wait for someone else's DVD to download. Likewise you don't form a long single queue to get into the football field. Feb 17 '17 at 11:43
• Long lines spilling out into the road is definitely one of the big factors driving this. In Phoenix a downtown McDonald's was torn down and rebuilt to accommodate the traffic issues. There are news articles about it online. Feb 18 '17 at 19:40

It's better for some situations like a quicker user gets through the line faster. Example: a car full of kids takes 3 minutes to order. A single person who knows what they want takes 30 seconds to order. By having two lanes, that single person has the ability to bypass the car full of kids. They get their food faster with minimal effect on the car full of kids.

• They could sign the right hand lane as the 'family lane' :-) Feb 16 '17 at 21:34
• This is like the benefit of having multiple processors in your computer. The greatest % increase is when you go from 1 to 2 Feb 16 '17 at 21:43
• If taking an order takes 2t and paying and handing out an order each takes t then the time saved for each customer is ~t. Feb 17 '17 at 9:13
• @PhillipW Because that works so well for "Carpool Lane" and "10 Items or Less?" ;) Feb 20 '17 at 0:21
• Depends on local culture. Here in the UK "10 items or less" generally works fine. We don't have carpools. Feb 23 at 8:09

# Time

Note: just like David Meister mentions in his great answer, this is based on Theory of Constraints

Let's say we have 2 rows of 4 cars and it takes 3 minutes to each car to make an order, plus 1 minute to pay. Since they're taking orders in parallel, it will take 12 minutes to each row to go through, plus 8 minutes for all cars to pay.

``````(3+3+3+3)+(1+1+1+1+1+1+1+1)
``````

That totals 20 minutes.

Now, think about the same scenario, only with just one drive-thru:

``````(3+3+3+3+3+3+3+3)+(1+1+1+1+1+1+1+1)
``````

it will take 32 minutes.

# User Experience

Same case, with just one drive thru. The first 3-4 cars will just wait patiently. The other cars will grow impatient, and maybe even try to get out and go to another restaurant, causing trouble to the cars behind or getting stuck in the middle. Needless to say the amount of frustration will skyrocket. If you want more drama, try to picture this with 3 kids yelling and you'll get the perfect nightmare (and obviously, this user will never get back, granted).

# Avoid friction

Same situation. Now you come with your car and see a long line of cars that gets to the road. Being smart, you say to yourself: is the time I will spend here worth it? Quite probably not, so you move out to another place.

# Maximize costs/benefits

Consider the cases mentioned in my first point. Now let's say you need 10 cars to pay your employees, and you have only one employee per station (taking orders and cashier). For the purpose of this example, you work at full capacity 8 hours.

So, with 2 drive lanes you have 3 employees, therefore you need 30 cars to pay the costs. As I mentioned before, it will take a total of 4 minutes to each car. But since they go in parallel, it will take 75 minutes to pay the costs, with an average of 2.5 minutes per car. Then you'll have 405 minutes of remaining time, or 162 cars (`405 minutes / 2.5 minutes = 162 cars` )

One lane case: you need 2 employees, hence 20 cars. It will take them 100 minutes (20 x 4 minutes) and leave 380 minutes of remaining time. Now `380/4=92` so you saved a bit in employees wages, but your income will fall drastically, almost to half.

# Prevention

Let's say you've a busy restaurant. Then, oh catastrophe, something happens. Let's say the ordering computer goes kaput. Or your employee fails to show up. So you lose sales until you fix the issue. By having another lane, you will prevent the chances of losses by (literally) 50%

# In short

It makes all the sense, it's all benefit with barely any downfall

• A point on "By having another lane, you will prevent the chances of losses by (literally) 50%": actually you double your chance of any loss, because there is twice as much stuff to go wrong; but we reduce the cost of any single failure significantly, and better than half the probability of the system falling over completely (this all assuming the event of either lane closing is independent from the other). It is interesting to read about the number of engines on aircraft (commercial and military), and how these considerations effect such choices. Feb 18 '17 at 13:16
• You're assuming there are multiple order-takers, so that orders can be taken simultaneously, but I have never seen this. In fact, I have never seen more than one booth in operation and that booth both takes orders over the speaker and takes payments at the window, further reducing the effect of parallelisation Feb 20 '17 at 18:29
• Your additions are wrong – the payment can happen in parallel to the ordering of the next customer, so it is 15 minutes versus 25 minutes. Feb 20 '17 at 19:32
• This answer lacks freehand red circles.
– Yakk
Feb 20 '17 at 19:33
• @PaŭloEbermann I'm providing a simple and easy to see/explain model, not a real-life model. Of course that could (and will) happen, but for the answer's sake, I simplified this to improve visualization of data. Yes, I could also use regression and statistical models, but... would they help anyone? I don't think so Feb 20 '17 at 22:58

Reduce time and/or reduce queue before the process starts.

The process goes:

• client orders
• restaurant prepares the order
• restaurant serves the order

If the restaurant places two lanes then two clients can start the process at the same time or at least make two queues. I guess seeing a queue of cars might make potential clients go away, so if there are two lanes for the first step you reduce the amount of clients that haven't started the process to half (and move the queue to after the order).

It is not the same to wait because one already ordered, than wait to order, because in this last case the client might simply leave.

About reducing the time, as others have stated, the first step is the one that doesn't depend on the restaurant and might take more or less time depending on the client.

In the case where only one person is taking orders, the time for each client would be the same as having one lane (no time reduced). However, there would be two short queues instead of one long (at the start).

• Since the two lanes are BEFORE the payment window, it doesn't really benefit the "restaurant" at all to have the 2 lanes. I could place an order, land at a cue, then bail before I've paid and AFTER I've executed the order. The only way a place like this would benefit from the 2 lanes would be if the user paid upon ordering because only then are they obligated to wait. Feb 16 '17 at 23:42
• @coblr well, you are right in that, the client can simply leave at any point before payment. But if the client already ordered, I believe it is more probable he will stay than if he didn't order yet. Feb 16 '17 at 23:46
• You missed the, "client pays," step, which in theory happens in parallel to preparation. @coblr I think you're assuming that the restaurant isn't willing to risk the loss of preparing food that the client never pays for. I doubt the risk is all that high; most customers pay and get their food. Fast food restaurants already budget in some lost food for things like mistakes in preparation anyway. Feb 17 '17 at 1:13
• @coblr: Actually, there's generally quite a substantial benefit to the restaurant, though it's not in forcing people to pay. The benefit they get is more people getting in line due to the appearance of the line being shorter. Most people going through drive-through could 1) skip eating right now entirely, or 2) move on to a different restaurant very easily. The longer the lines look, the less likely it is that other people will get in line at all. Feb 17 '17 at 1:58
• @coblr: How do you bail from a drive-through line? Unless there's nobody behind you, you're boxed in on all four sides, so you have little choice but to keep moving forward. Granted, you could just refuse to pay, but why would you after waiting that whole time?
– user69458
Feb 17 '17 at 17:44

I think another way to look at it is by an analogy:

In the men's restroom count the following:

• number of urinals
• number of handwashing faucets
• number of paper towel dispensers
• the in / out door

Just because you see 8 urinal stalls does not mean one also has to provide 8 doors, correct?

The underlying point is that you need to provide more parallelism at the slower steps. Assuming, and I think it is a good assumption, that ordering takes more time than paying it makes perfect sense to have more ordering-windows than paying-windows.

You see this same feature in tons of, at first glance unrelated, user experiences:

• Lanes fan out at toll payment points on the highway. You may have a 2 lane tollway but 8 parallel toll booth lanes.
• You may have 10 self check in kiosks but a single baggage drop point
• You may have a dozen manual check in counters and weighing scales shared between two agents but only a single conveyor finally leads to the baggage sorting room.

Car pulls up and car reads menu - that is 100% dead time for the order taker.

It has a higher loading of taking order with the same manpower. Hence more orders process in the same period time.

Both faster and more efficient.

the payment and receipt of food is still done serially, which would seem to negate any benefit from orders being placed in parallel.

There is no reason to assume this is true.

The answer, from a UX perspective has to be:

The benefit for the user is getting their food faster because the restaurant can process more meals per hour, leading to shorter queues.

There are two things to consider here to understand why the basic premise of this question is probably wrong.

Simply moving from a concurrent step to a serial step does NOT negate the benefits of the earlier paralellism.

Let's look at an analogous example from web development. We want to fetch data from two different servers (parallel processes) and then use that data to render text into a web page from top to bottom (serial process).

If it takes 3 seconds to hit each server and 1ms to render the response from a server we can obviously either take 6 seconds or 3 seconds to finish the task depending on our choice of parallel or serial for the first step ONLY.

The second step's parallelism changes the overall situation by 1ms, making it completely irrelevant.

You see the same situation at the fast food restaurant.

Time to take an order, create and package the food ~= 5 minutes.
Time to take money and hand food through a window ~= 30 seconds.

Provided that the internal team has the facilities to prepare at least 2 orders at once, the result is:

Step 1 in parallel = 5.5 minutes total
Step 1 in serial = 10.5 minutes in total

The parallelism of step 2 doesn't negate anything here.

A system with a series of sequential steps is constrained ONLY by the slowest step.

This is called theory of constraints. It has a wikipedia article too.

The summary is that in a system with a chain (or even multiple converging or diverging chains) of processes that need to be completed, the system moves only as quickly as the slowest step.

Attempts to optimise any step other than the slowest step have no positive impact and can even have a negative impact on overall productivity.

Imagine a traffic jam at peak hour where many lanes (A) converge into fewer lanes (B) and then diverge back to many (C). Something like this:

A - will make traffic worse by increasing congestion ahead of the bottleneck (incidentally, this is why we use words like "bottleneck" to talk about a limiting factor in a process)
B - will increase throughput of the system
C - will have no positive or negative impact on the traffic

Your example of a fast food restaurant (or literally any other system in the world) has just one severely limiting factor at any point in time. Nothing else is worth optimising.

Think of the following stages of throughput in real space/time terms of burgers per minute per square meter, after all, this is how they pay rent (I'm just guessing rough numbers):

• Collecting orders (~ 2 mins) from a drive thru lane (~20 m2) = 0.025 b/min/m2
• Preparing orders (~ 3 mins) from a burger grill (~1 m2) = 0.3 b/min/m2
• Delivering orders (~ 0.5 min) from a kiosk (~2 m2) = 1 b/min/m2

Now also consider that the nothing can be done before you place the order yet preparing and delivering orders can be done in parallel with taking new orders.

It should be clear why:

• A very large amount of physical space shown in the OPs aerial photo is dedicated to taking orders, relative to other tasks
• Taking orders is handled in parallel (it is the step that needs to be optimised)
• Every other step (before and after) can be safely handled in serial (trading throughput/latency for less physical space requirements) as their throughput does not effect the throughput of the system overall

It also hints as to why perhaps having 2 lanes for orders makes sense but not 3, or 10 or 50 - this would be out of proportion for the size of the kitchen and other available facilities required to process burgers.

One potential avenue for followup, would be whether this restaurant is really optimised for consistent user experience (latency) or just total revenue per square meter (throughput). As far as sending items concurrently through parallel/sequential processes goes, they are not necessarily the same thing. It's quite possible to optimise overall throughput in such a way that individual items may suffer additional latency (like when the OPs order is forgotten at the kiosk).

This is a pure UX question. I did similar research for bank teller lanes in college. I will answer your question with one word - perception.

When you drive up and see 5 cars ahead of you, you might be like "F that I will try somewhere else." They know once you order you will hardly ever leave. So they just want you ordering and don't want to lose your business. Your question has two lanes in the example... Sonic took this to the max as some of their locations have 40+ ordering points. It is all about perception as you are still in line. They just want you to think that the line is as short as possible until you order.

I am not saying the other answers are wrong. They make valid points but they are all secondary to this point. As you can see below, you can drive up and order right away at Sonic and you will be the first person in line ;).

• I already said what you said, but I fail to see why all other reasons are secondary to yours. Besides, perception isn't created in a vacuum, but it's the resultant of all the stimuli the user receives... namely, everything you consider secondary Feb 18 '17 at 21:10
• @Devin - No you didn't - at least not in a coherent way. You are talking about queue times and how long it gets someone through. I am talking about the perception of time a customer feels like they will be waited on. In the two lane example we did numerous studies and unless there is one staff member per lane the time savings is often negligible as there will be an increased rate of mishaps. Now Sonic has taken this further because they make it seem like you are waited on right away AND the consumer has no ability to calculate length ... cont. Feb 20 '17 at 1:32
• cont... because the consumer has no idea who has and hasn't ordered. In the two lane example it is pretty easy to calculate how many cars there are ahead of you... and 3-5 mins per car... so you can get to it. But places like McDonald's basically think that their average customers will not go through these calculations and if they do they would have already spent time in line and will not give up on a sunken cost. Feb 20 '17 at 1:35

Pure speculation but it makes sense.

Payment and receiving food are very quick. What takes the most time in any food place, including fast food, is food cooking/preparation. If they can get multiple peoples orders they can get all the food preparation out of the way and simply trade food for money at the window.

• Actually as @Josh Carr mentioned in his answer, The most lengthy process could be the ordering of the food. My instinct has always been that with fast food most of the food is already prepared and is cooked very quickly Feb 16 '17 at 21:42
• @yitzih it's certainly possible ordering could take longer, it will depend entirely on the order. I could just drive up and say I want a #10 and be done ordering in seconds or I could 5 kids in the backseat and take all day. At the same time they could shove some chicken in a bucket in the prep process or prepare two dozen tacos. The main point is that the window isn't going to be the bottleneck so it makes sense to have multiple lanes. Feb 16 '17 at 22:00
• @yitzih ...or it's all already cooked and the just have to throw it into a bag. Unless, of course, you have a special order, in which case the person at the other window gets their food first (the cashier may ask you to pull into a parking spot and offer to bring your food to you)
– A C
Feb 16 '17 at 22:56

Having worked in a McDonalds I'd say the main advantage of 2 windows is so that the orders are placed and they can start being made. It just means more orders can be placed faster. "the person at the payment window repeats the order to each driver to confirm that the orders/drivers are in sync", I know in England, the staff taking the orders get a video feed of the customer, this automatically takes a photo when the first option is placed on the order. The person at the payment window then gets a face next to the order to confirm who placed it. This doesn't always work because the pictures aren't always perfect but the majority of the time it works well.

The staff member taking the order must.

• Wait for the preceding customer to drive off
• Wait for the new customer to drive to the order “window”
• Wait for the new customer to wind down the car window
• Take the order
• Repeat

Therefore it is very possible that one member of staff can operate two order windows, processing more customers, then if the customers were all at the same window. If the lines start to back up, it is very easy for a 2nd member of staff to step in for a few minutes, so it is then one member of staff per order “window”.

At the collection window, the member of staff has to:

• Find the correct food on the rack
• Get the food from the rack
• Put it in the bag
• Get the chips
• Take payment
• Give the food to the customer

Often this is done by one member of staff, however it is easy to see how the work at the payment and collection window can be split between two members of staff. Most of this work can be done while the preceding customer is getting out of the way and the next customer is driving to the window, so unlike the order window, the member of staff is not spending much time waiting for the customer.

Just a comment that I didn't see addressed on this with all the perception/frustration commentary: Merging can be a stressful and annoying process.

When you leave the double line and merge back into one, many drivers are paying vary close attention to the proper order of things, who came first into both lines, proper zipper merging when both cars are waiting to move forward, jerks, etc. Or simply having to make a choice and being stuck in the slow line after choosing.

While not scientific, both myself and the person next to me at work sometimes prefer to avoid busy double line merging drive-throughs just because it adds stress to the process. More than potentially waiting longer elsewhere, even if the lines does move faster in absolute terms.

Also another interesting variant: The McDonald's near us has two order windows, BUT they are in sequence not parallel. So it batches two orders at once, but the line order never changes. (And the "back orderer" can still hold up the line of course, with an empty front order slot...or someone that just doesn't pull up.)

1. profile the situation and identify the bottleneck: ordering at the speakerbox takes a long time, much longer than it takes to get the meal and pay at the window.
2. add more capacity to that bottleneck: double the number of speakerboxes.
3. profit!!! (there is no ? step)
4. repeat from step 1

A lot of verbose answers here. The simple and short answer is that queueing theory tells us that multiple queues will reduce variance in wait times, while the average wait time will remain the same.

Predictable wait time is always preferable in terms of UX, and although it's not the only factor, it also is better on the production side (imagine one person asks for a bottle neck item, and your entire production chain is blocked until you can deliver that item).

Having read all the answers so far, I feel like a mathematical approach can be of value.

A waiting line/queue is typical version of a Markov-chain. A Markov-chain is a network of states. For a waiting line, this are usually the nr of customers in the total system (the nr of customers waiting in line plus the nr of customers being served). States change according to a certain probability which is different for every state (transition probabilities).
How can we use these probabilities? lets use the following example (Ross 4.8):

Four day weather forecast

These probabilities are usually described by a Poisson process. A poisson process is a mathematical representation of probability over time (note that this is not entirely true but it does give an idea of how one could approach it). In other words: how many of a certain event do I expect over an amount of time. For instance: How many car accidents do occur on a certain junction in 10 days (If that is 2 that may seem very high but since there can be many cars passing, the probability of an accident happening when one passes the junction can be very low). A more on-topic example is, how many customers do I expect to arrive to the drive thru lane? This immediately brings up the question how much time will it take before the next customer arrives. We call this inter-arrival time.

One can see that the values from P^2 to P^4 change less than the values from P^1 to P^2. What does this mean? Can we use it? Lets assume the values in matrix P^1 represent reality. Of course it doesn't, but lets assume it does. The truth is it converges to a point which is not dependent on the state of today anymore, because, the further in the future you go, the less today is of importance of the future. This describes the memory less property of an Exponential probability distribution. To find these so-called steady state probabilities, one should calculate the matrix P^Infinity. Sometimes this is easy, sometimes it's not because not all matrices/probabilities converge. Some matrices diverge. An illustration to this: What if the average interarrival time to the drive thru is larger than the average drive thru's service time, and the drive thru would be of infinite length? The system would "blow up" meaning that waiting times will only grow larger. In reality this is never the case since a drive thru has only a certain length.

what can we do with the steady state probabilities? We can calculate the average waiting time of a customer in the system. We can calculate the amount of numbers expected to be in the system and we can calculate the expected utilisation of an operator (and many more things).

Lets say a company adds a lot of value to service. Waiting is often regarded as not adding value to service, since, of course people don't want to wait. The company can now calculate the costs/benefits of having an extra operator/drive thru lane and see if that adds value to the company. Maybe the company now gets more customers since even though there are more customers in total, the total waiting time is lower.

Something I'd also like to add is: I once made an analytical (markov chain) model for the parking garage of a hospital. This hospital had 2 entrances, one on the north side and one of the south side. The hospital asked me why, even though they had 2 entrances, people were still complaining about the waiting time. What was the case is that no one was using the north entrances because not many people arrived from that side. It is often better to have one central waiting line that separates into multiple servers.

From the customers' view it is often better to stay in your line: https://www.youtube.com/watch?v=xvQjTvktTKI

Making the food is the slowest step, so the fastest process will include telling the kitchen what the custumer wants as soon as the they arrive. Then the payment can be done at any pace because food will take longer to get ready. (supposing that the restaurant starts cooking before the payment is confirmed -- not always the case)

Having two lanes only for the ordering step is the same of a person walking through the queue asking what they will order instead of waiting them to get to the cashier.

And plus, if people order soon, they feel they haven't had to wait that much.

Have you ever ordered your food at a drive through, paid at the payment window and then been told:

"Your food isn't ready, park up and we'll bring it out to you when it is"

By having two lanes, orders are being taken twice as fast but everything else remains the same. Which means 2x more food can be cooked in the time it takes each car to get from entrance to collection.

a lot of great answers here. From my experience in drive thru queues for fast food, i believe it boils down to the following

Multiple queues looks shorter than it's combined length
a long queue can be demotivating to someone who wants to join the the queue. It would give an impression of a longer waiting time. This solution would counter that effect.

Illusion of choice
When there's multiple queues, customers will decide join the shorter queue in the hope of a shorter wait.

One thing everyone failed to mention...

The advantage of 2 (or multiple) POS over 1 depends on if there’s 1 long queue that feeds to multiple POS or if each POS has its own queue. The former is far more efficient for the user.

If the user waits on one queue until they are next, and at that point they are waiting for the next available POS, they get the full advantage... if one person at the POS is ordering \$50 and at the other POS, ordering a drink, the user waiting waits until the first POS becomes available and then moves up. It totally negates the issue of getting stuck behind someone ordering \$50 worth of stuff, the next 5 people can move through the other POS while the one is tied up.

With multiple queues, the user, when they approach, has to pick one and take a gamble. With 2 queues, there’s a 50% chance they will pick the wrong one, the one where the person in front of them is ordering \$50, while they watch 3 or 4 people move through the other POS/queue.

So with one POS, one queue, the probability you will get stuck behind a block (a \$50 order) given one exists ahead of you, is 100%. With n POS and n queues, the probability that you will get stuck behind a block given one exists ahead is 1/n, n POS and 1 queue, the probability you will get stuck is 0.

My McDonald’s has 2 POS’s and 2 queues, so they have only reduced the probably try of me getting stuck behind a slow poke by half. All else being equal, half the time I will pick the faster lane, and half the time I will pick the slower lane, so they improved half of my user experiences.