In general, if you're driving somewhere remote and something goes wrong, you're almost always better off staying with your vehicle vs. setting out on foot for help. There are obviously exceptions, but as a general thing, it's true.
That reminds me of the sad story of CNet editor James Kim. He got caught in a snow storm with his family. He left the car and died trying to get help. His family was rescued in good condition. Had he stayed with them, they all would have survived.
The "family gets lost, guy goes off to find help, gets even more lost and dies, and then the rest of the family is rescued" is a pretty standard story.
Today, in the age of gps enabled cell phones there is no reason to leave the car (unless there's a risk of explosion, car sliding off a cliff, etc, etc)
Get your position, call 911, wait.
If the signal is not good enough to make a call, send an SMS
What if the signal isn't good enough to send an SMS?
I'd tend to agree that staying with the car is sensible. Maybe with some bright orange tarpaulin / flag thing over the car (although that'd probably get buried in snow.)
At least if you're travelling in really bad weather you know you might get stuck and so you should prepare.
People in England were caught out when light snow was forecast; they left work early and joined the M25 at the same time as the road gritters, which caused a weird cascade effect and blocked a major motorway in the UK for 24 hours when there was only 1 or 2 cm of snow.
Then write your SMS, try sending it and try exploring different positions (maybe try exiting the car, or even climb it). Just don't wander away from it.
In my experience, the phone will retry sending the SMS a couple of times if the signal is low.
When I go riding around central Utah on my motorcycle (note: these are paved roads with a street motorcycle), I'm often out of cell coverage for hours at a time. The reality is that the high mountains and deep valleys for "real" mountains, combined with the lack of towns, provide for no coverage, either SMS or cell. If I got caught in those places in snow that is feet-deep, I wouldn't expect my electronics to free me.
I would probably be more likely than most to leave my vehicle but I have training in mountaineering and I wouldn't do so without a good plan and sufficient equipment. My car always holds a basic level of backup supplies including: a first aid kit, a bottle of water (which can also serve as a water container), a head-lamp with extra batteries, a multi-tool, matches, a fleece blanket, rain gear (including rain pants), mylar thermal blankets, and a simple drawstring backpack (which holds the blanket and rain gear) in blaze orange.
Even in a worst case scenario where I happened to drive up into the mountains in just jeans and a t-shirt and get caught in a snow storm and nobody knew where I had gone I could easily travel for several days through the back country (let alone alone the roads) with just that equipment. And if I stayed with my car it would definitely extend the amount of time I could survive until rescue arrived. The total cost of all of this stuff is almost nothing, it's worth it to be prepared.
I don't really leave a 2hr radius of SF now (although I guess I drive to Front Sight and Vegas sometimes, but prepare separately for that), and have all that stuff and more. Case or two of water, some tools, often a range bag with items, etc. Mostly worried about responding to someone else's accident, but there's the lingering fear of an earthquake, big fire, or other event like that. This is one of many reasons I like having my own car vs. zipcar or public transit -- it's 100 pounds of things to deal with emergencies (including "server broke, need tools and some network cabling", which is a much more common emergency for me than gunshot wounds...)
I really want to get into bay area amateur radio and get on some repeaters and stuff. An HT in the car seems like it would be a good addition. Maybe 2013.
In general, if you're driving somewhere remote and something goes wrong, you're almost always better off staying with your vehicle vs. setting out on foot for help. There are obviously exceptions, but as a general thing, it's true.