I think most of the commenters here miss point of the sneckdown. It isn't to "prove" that all of these places don't need to be roads, it's merely a way of illustrating potentially unused road spaces. A hint, not a proof.
Having said that, I do think the concept only works well for relatively light snow, the amount of snow that most drivers wouldn't think twice about driving over. Big piles of snow are of course going to be seen as an obstacle, something to avoid, while an undriven area with just a bit more than a dusting can genuinely tell us places that drivers might not be using at all.
