The 10 seconds is the rule of thumb, according to the California Energy Commission, because 10 seconds of idling can use more fuel than restarting the engine, so after that point, you are wasting fuel.  For every two minutes a car is idling, it uses about the same amount of fuel it takes to go about one mile.

While it may not be as practical to turn off your engine at a stop light, unless you have severe heat or cold conditions that require you to leave the car running, why not turn it off.

The CA Energy Commission also point out three other myths associated with idling:

Myth #1: The engine should be warmed up before driving. Reality: Idling is not an effective way to warm up your vehicle, even in cold weather. The best way to do this is to drive the vehicle. With today’s modern engines, you need no more than 30 seconds of idling on winter days before driving away.

Myth #2: Idling is good for your engine. Reality: Excessive idling can actually damage your engine components, including cylinders, spark plugs, and exhaust systems. Fuel is only partially combusted when idling because an engine does not operate at its peak temperature. This leads to the build up of fuel residues on cylinder walls that can damage engine components and increase fuel consumption.

Myth #3: Shutting off and restarting your vehicle is hard on the engine and uses more gas than if you leave it running. Reality: Frequent restarting has little impact on engine components like the battery and the starter motor. Component wear caused by restarting the engine is estimated to add $10 per year to the cost of driving, money that will likely be recovered several times over in fuel savings from reduced idling. The bottom line is that more than ten seconds of idling uses more fuel than restarting the engine.