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?
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.
Reduce time and/or reduce queue before the process starts.
The process goes:
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).
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.
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.
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?"
Note: I don't have graphical software in this laptop, so you'll need to picture the scene. Or even better: draw it in a sketchbook or sheet of paper for proper visualization.
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. That totals 20 minutes.
Now, think about the same scenario, only with just one drive-thru: it will take 32 minutes.
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).
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.
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.
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%
It makes all the sense, it's all benefit with barely any downfall
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 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.
Anyone notice the pedestrian trying to order?? Maybe the extra lane is for all those cars they will serve (whilst the ped argues)!
Anyone notice someone has broken the height limit bar? Maybe too many trucks and buses have tried to use that McDonalds and they needed a spare lane?
FYI - I had a bus load of kids and only the one single drive-thru lane was open - I offered to block the lane so the kids would be safe, but they still wouldn't let us order.
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.