Found an interesting article on Slate about whether to idle or shut off you car. It even goes into whether to warm up your car or just start driving.

Some excerpts:

Today's cars use electronic fuel injectors, which rigorously control the amount of gas delivered to the engine when you hit the ignition. As a result, virtually no fuel is wasted during startup, and only a thimbleful is burned as the car roars to life. So forget about the 30-minute axiom you were raised on - the threshold at which it makes more sense to shut off rather than to idle should be expressed in seconds, not minutes.

The researchers concluded that restarting a six-cylinder engine - with the air conditioner switched on - uses as much gas as idling the same car for just six seconds.

Idling is similarly wasteful in frigid temperatures. Contrary to popular belief, cold-weather drivers needn't warm up their cars for longer than 30 seconds. The best way to raise an engine's temperature to optimal levels is to drive it almost immediately after startup; according to a study by the Ontario Ministry of Transportation, a car driven for 12 minutes in 14-degree-Fahrenheit weather will achieve the same temperature as one that idles for 30 minutes. (However, it's best to avoid rapid acceleration during that 12-minute warm-up drive.)

But if we were able to eliminate idling in stop-and-go traffic, the effect could be more dramatic. Right now, it is imprudent (and often illegal) to cut your engine while on public streets. There are automated systems, such as in the vaunted Toyota Prius, that can rapidly turn engines off and on when the car is, say, stopped at a red light or involuntarily "parked" on a bumper-to-bumper freeway; just apply some pressure to the accelerator, and the engine springs back to life. According to the learned folks at Car Talk, the widespread adoption of such technology could reduce our national fuel consumption by as much 10 percent.

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