In a blizzard, the snow will often cover the vehicle for you. It's not a bad idea to add more. I think you would need to have a foot at least to gain substantial insulation value, and three feet would be better. Also, don't discount the gains of building up a windbreak and loosely wrapping the vehicle in whatever tarp material you may have.

Signalling is important though. A high-speed plow can total a vehicle. I always carry a few of those orange flags from building supply stores that are used for long loads (hey, they're blaze orange, multipurpose, and free). I also have one or more bicycle flasher lights, the red LED jobs, in each car. That preserves the battery, and you can mount the flashers on a stick or a tripod on the roof. There's nothing worse than coming back to retrieve a vehicle and finding the battery is dead and destroyed due to freezing.