I was at Rainbow Beach Resort last weekend when I saw a man park his truck in front of the office and go inside. That, clearly, is not exceptional. What was exceptional was that he left his truck running, and he was inside for 10-15 minutes. My first thought was what a waste it was to leave a car running like that. But my next thought was "How long is too long to leave my car running? Should I turn it off if I'm idle for one minute? Two minutes?"
I went online searching for the answer, and I came across an article in Slate Magazine. The author, Brendan Koerner, did some research of his own, and determined that I should turn my car off if I will be idle for only ten seconds. That really surprised me. He estimated that adhering to this rule (except, of course, at red lights) would reduce the amount of carbon dioxide emissions by over 15 million metric tons per year.
He also noted that, in the winter, it is wasteful to idle our cars to let them warm up. It is much better to drive it immediately after starting up, but to avoid quick acceleration during the first 12 minutes of driving.
You can read his reasoning for his claims here.