I don't see the plan B unless it was the cell phone.
Plan B was having several days of food and water, and a way to survive freezing temperatures for days.
I tend evaluate these things in terms of "did they assume things would go right?" This is not usual idiot case since they did _not_ plan for everything to go right - they had food, water, etc, in case something went wrong.
They drove 25 miles in snow without seeing any signs of civilization and thought this was a good road??? I knew better than that on my first road trip at the age of 19.
It was a good road for less than a mile. I can't tell from the aerial picture if it's still paved after that but it is narrower. I can't tell where the warning sign is or where the first "Forest Service Road" designation is.
But within a few miles after that it clearly narrows down to not much more than a one-lane path, crowded by trees. They're got to have traveled 20+ miles with the *trees* telling them this is wrong, even if they can't see ground boundaries because of snow-cover.
Another thing is that the aerial photos show that there are many such roads in the area intersecting each other. Even if they decide to turn around and can't right there, an intersection is likely within 3-5 miles. They aren't much, but it's enough room to turn around.
I don't fault them for turning onto this road because it looked like a main road where they turned. And they might have missed the sign (given the condition of the road, I doubt much more effort goes into signage). But certainly within 5 miles it got to be obvious this is a one-lane dirt path, not a small county road between towns.