So apparently if you own a car with fuel injection, it is better to stop the engine, but if you have an older car with carburetors, it's better to leave it running.

By the maths of the article below, cutting out 10 minutes of idling a day, 5 days a week, all year long, will save you about 27 riyals worth of petrol. Per year.

Wow.

http://www.slate.com/id/2192187/