Experimented with a route I've refined through the course of the winter. Google found a "trail" kind-of parallel to my route, and every time I tried to drag it off, there was a different part of the route bouncing back. Said trail is covered in 3-6" of snow, just now, even though the roads have been dry for the better part of a week. When it thaws it will be mostly dirt with some wood-chip "improvements". A simple check-box that says "avoid unpaved routes" would help.