The amount of gas it takes to start up is about seven seconds worth of idling gas. So if you're going to be sitting for more than seven seconds, it saves gas to shut down. The argument is the wear on the starter and the possibility of older cars not starting back up everytime. Personally, I have an older car and I don't want to spend extra money on repairs, so I only shut down if I'm certain I will be sitting for over two minutes. But some people have their engines off while approaching the red light.
