HERE ARE SOME THOUGHTS ON ROADSIDE SURVIVAL KITS, from SayUncle. And there's a lengthy discussion thread on the topic at Knoxviews. I carry something similar to what SayUncle lists -- and I highly recommend including tampons or kotex, good for treating wounds as well as the obvious, and a roll of toilet paper, good for, well, the obvious. We keep some "Mainstay Emergency Ration bars" and water in the car too -- advantage being that they are so unappetizing that we'll never eat them unless it's a real emergency.
At the very least you should carry something like this or this in your car. None of that will prepare you for a James Kim kind of situation, though,
But it's worth noting that your car itself has a lot of things you can work with, including gas in the tank, lines, and carbueretor (if it has a carbueretor). That should let you build a big attention-getting fire. I've heard of people freezing to death on a broken snowmobile with a nearly full tank of gas -- that kind of thing shouldn't happen, and it when it does happen it's usually because people, under stress, don't think clearly. That's where mental preparation is as important as buying supplies. (This is a good place to start.) More on the subject can be found here.