If you are doing this in conjunction with a car, then the car is your shelter. It is superior to anything else you can carry.

Except for unusual circumstances, you are safer using the car as shelter than walking out.

+1; stay in the car. As for which bivvy, either the one I suggested in my previous post, or the slightly more expensive one suggested by AKSAR.

When you are in the car, you are out of the wind and wet: only only have to deal with the cold. This simplifies things greatly.

emergency bivy or bothy bag

Again, I must confess my ignorance. I'm well aqquainted with bivies, but what is a bothy bag?

I once saw a site where they had some interesting projects for used space blankets. And in another forum I saw mention of survival suits made from space blanket material. I don't remember where they are sold however.

I assumed that was just a typo, and they meant "body bag". But if there is this bothy bag thing, I'm interested to hear what it is too!

It is essentially a fabric bag large enough to hold three or so people, crowded together for sure. No floor, but some models have windows. Think of a small tent without poles.

Bothy bags are a Brit thing. Lord knows they have the weather that justifies such things.

Link: https://www.youtube.com/watch?v=EHmfxOBB3Ug

Consider the Gatewood Cape - dual function poncho/rain gear and shelter. I have no personal experience, but I hear good things - http://www.sixmoondesigns.com/tarps/GatewoodCape.html

Thanks hikermore.
Having seen a bit of the video I remember seeing them before. I don't like the idea of no floor especially in wet or cold weather.
A long time ago Outdoor Life or Field & Stream reviewed a poncho, tent, coat, etc device. I tried to track them down but the company had simply vanished.

I watched that video link. It looks like if you enter your "bivy bag" backwards (head first) and squat instead of lay down, you can rename it a "bothy bag". Seems like an odd thing to do. I don't get it.