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Idling uses gas, but because the car is going nowhere, it translates into 0 mpg. An automobile may burn more than half a gallon of fuel for every hour spent idling, so turn your engine off for long stops. How long is long? As a rule, shutting off your engine for any stop anticipated to be longer than 30-60 seconds saves gas and reduces carbon dioxide emissions. But make safety your highest priority, and only shut off your engine in situations where you are not in traffic, such as waiting to pick up the kids or when you’re making a quick drop off or pick up.

Source: EcoDriving USA