People look at me like I'm nuts because I turn off my engine while waiting in line at a drive-through. I've been told that if you are going to be still more than 20 seconds, you're better off turning your car off. Yes, we have to wait longer than that at stoplights, but in most places, it is illegal to turn off your engine at an intersection. So I'm saving fuel waiting for Taco Bell to fill the orders in front of me, probably incorrectly, while the SUVs around me are roaring the whole time. This guy explains how the formula is determined, which involves some math, but stick with it. Turns out that 20 seconds was a generous guess.
It doesn't save super much, it's in the range of 0-2 percent if I recall correct.