The idea is simple. Once you have ordered your food at the window, it is your obligation to move your car as far forward as possible so that the person behind you has the opportunity to place his order. Once the car in front of you moves forward, you move forward again, the person who was just ordering moves up for the person behind them, and round and round the glorious wheel of human dignity spins. What you must never do, is to sit idle while the car ahead of you moves forward. This is a logistical nightmare, and only causes the wheel to slow down.
You may wish to rebuke this claim I make, by stating that "you would be waiting anyway since they are still ahead of you".
Untrue.
Let's assume that the average time, for an average order, is represented in minutes, I will call it X. That would mean, I am the only person in line, I order, and it takes X minutes for them to prepare my food, have me pay, and send me on my way. What the Drive-Thru Obligation does, is allow the person behind you to get their order in before you get your food. So while the first person gets their order in X minutes, the second person may get it in X + Y minutes. This is achieved by the restaurant working on more than one order at the same time, with Y being an indicator of the time the first person took at the order window with person 2 waiting behind them.
Why is this significant? I'll tell you why. The idle sitter (as mentioned above) delays the order of the car behind them (sometimes 2 or more cars behind), moving the whole process closer and closer to a rage-inducing one order at a time scenario, whereby the restaurant could be making my food ready, but has to wait for the jackass in front of me to pull forward so they can hear what I want. This changes the equation to include another variable, changing it from the X + Y scenario to a less defined X + Y + i, for the indefinite minutes involved (i stands for inconsiderate jackass). In essence, i is the designation of time wasted for the driver behind the idle sitter.
There are concessions that one makes at a drive-thru. When you pull up, you know the establishment (generally) and you know how long it takes them to get their food ready. Another concession you make is how many people are ahead of you. You may even make concessions regarding the size of the vehicle in front of you, and how it may make for an awkward sequence of patrons waiting for their food, or worse yet, that one of these giant cars may be ordering for a whole gangload of people you can't even see. You make these concessions because you know that all of these were conscious decisions by other people, not the result of a careless or unknowing person.
I realize that someone reading this blog might be one of these very idle sitters I am talking about and not realize it. They may not know the pain they cause people like myself, who see such things and whose thoughts alternate between the collapse of human society and taking a lead pipe to the headlights of the car in front of me. We don't actually do these things, of course, because that would be the same de-evolution (not sure if that is a word) of the human race as your carelessness. There are a million other things I would rather spend my time doing than wait for other people to get their food, so there is no reason any of us should wait any longer than necessary. I only have one promise / request for the lot of you...
We will move up for you, just pay it forward.