I like multi-tasking equipment. In my car I have, rolled up tightly together and stored in one of those nylon bags that comes with folding lawn chairs:
- two (2) military-style ponchos, with grommets and snaps
- one (1) poncho liner
- one "sportsman's blanket" which is poly on one side and metallic "space blanket" material on the other, also with grommeted corners.
With these (and a few feet of paracord) I find I can improvise a pretty snug and waterproof shelter/bivvy/sleeping system just about anywhere. I also have the flexibility of using the components of the system as rain gear, equipment covers, for signalling, etc. If I find myself with a couple of other people, we can split up the components so everyone can at least stay fairly dry.
I don't recommend this system as your primary shelter on a planned camping trip. It's also a little bulky, though not terribly heavy. But in an emergency I know I can sling that bag over one shoulder, and my medium-sized emergency kit (also in the vehicle) over the other, and be reasonably prepared for a few days and nights in most conditions.