When you were first learning how to drive a car, did you receive a particular warning from your instructor, or perhaps a friend or familiar member? Specifically, did they warn you not to leave your car idling for long stretches of time, as it would unnecessarily burn up fuel from your tank? If nobody has clarified for you since then, allow us to be the first: it's quite true.
Leaving your engine idle for long stretches of time, whether you're just lazing about in your driveway waiting for the engine to warm up or stopped in gridlock traffic on the highway, will very gradually drain down your reserves of gasoline. While idling isn't likely to completely drain out your tank at the same rate that regular driving does, it can have a tangible impact on your overall gas mileage, and according to studies, the many drivers of the United States actually lose billions of gallons of gas to unnecessary idling on a regular basis, making it one of the least fuel-efficient things you can do in your car.