There was a fire call a few nights ago, one district over.
A garbage truck went off the road, into Lake Nockamixon. Both the driver an passenger survived, however, it was snowing hard, very very cold and miserable.
They had no cell phone or they had lost it (unclear), but they were wet, and they had climbed onto the roof of the garbage truck and were screaming for help for a long time. They were about 40 feet from shore, in about 7 feet of water.
Now, going onto the water is a Bad Thing no matter what, but it also points to the need to keep Things That Should Always Be Dry in a drybag, even when stored in your vehicle. In my Jeep, I keep a wool blanket, a space blanket (Wrap in wool, then space blanket), Hothands hand warmers, some dry shoes and socks.

At this time of year, you should also have a "cooler" in your car, and use it eskimo style - that is you use it to keep the heat in, not out.

A couple "Hot Hands" large handwarmer tossed into a small cooler will keep it just above freezing inside, which is a good thing for storing water if you're going to be parked overnight and can't bring water indoors. Of course, storing your water in the passenger compartment, rather than the trunk, is a good idea in the cold weather, but it may be a bit of a pain to deal with.

Since I have the room in my truck's storage compartments, I keep all garments and blankets in transparent vacuum Space Bags. It protects them from dust, moths, and moisture, yet I can easily determine what is inside. This prevents me from going through my wife's clothes while looking for coveralls to assist someone stuck in the snow. This increases efficiency and keeps me from being verbally abused.