If you are stuck in your car, you can leave the engine running if you a) periodically check to make sure the tailpipe is clear and b) you keep one downwind window open a couple of inches. In fact, if you use ANYTHING in the car the produces flame -- Sterno, candles, a propane heater, a campfire, whatever -- ALWAYS ALWAYS ALWAYS leave a window open a bit because these WILL produce carbon monoxide and that WILL kill you.

Another reason to open the window a tad is because of moisture and CO₂ buildup. The carbon dioxide can kill you just as dead as its monoxide cousin. Moisture can freeze and make it difficult or impossible to open the windows or doors.

And yes, the handwarmer packs do work but it you're not careful they can burn you. Use them with discretion.

999 isn't quite correct: Commons Sense isn't just uncommon, it's downright rare.