I've been following this story from the beginning and have yet to hear why they got off the I-5 to begin with, especially in snowy weather. I saw one story that said they were supposed to stay in Gold Beach the first night. Were they trying to take a shortcut over the mountains instead of taking route 42 and following the coast?
The speculation is that Google Maps or Mapquest or something else, without regard to local conditions, generated a map which would do that.
As I understand it, that's part of how they eventually located the car.
From what I read, they family intended to take I-5 to Highway 42 to Gold Beach. They missed the exit for 42 and instead of doubling back to it, they looked at a map and saw Bear Camp Road which looked like a good direct route to the coast so decided to take that. Apparently, on the map it looked like a regular road and they didn't realize the danger in winter.