This would probably be quite difficult and expensive to implement on the Make Me a Route tool, as there are fewer unpaved roads and paths than paved ones, so most of the routes generated would not qualify.
I see it's an old post but when I create a cycle route and choose the 'advanced' option I have two options with terrain: 'Any' or 'Paved'. So it is possible for 'plotaroute' now to have that choise, why not the choice 'unpaved'. I understand that it's most of the times not possible to make a 100% unpaved route, but would be nice to have a route with as much as possible unpaved. Plotaroute knows these sections as I can click on them to create my route manualy and it navigates nicely over it. I took a premium account because I was thrilled by the features to create a route, but the 'make a route' is a little disappointing now as I can't choose 'unpaved' to male my MTB- or hikeroutes.
Probably not in the forseeable future as the routing services we use don't have that capabiliity, but if that changes we can certainly consider it.
Admin.. Will this feature be added in later? So far this is the easiest most efficient mapping service I've used to create trails for a 4x4 offroading group..
Sorry, there isn't a way to do this.
Is there any way to only select gravel/unpaved or seasonal roads, or a mix there of?