You must have some mighty remarkable roads then -- in the rest of the world, they have weather-induced damage, as well as damage caused by nature even if it's not exactly weather.

As for weather related damage, rain can cause erosion, which damages roads. Cold days can have rain get into the road and freeze, cracking it. (In Oregon I wouldn't expect too much of that, but it shouldn't be unheard of.) Alternating hot and cold days probably weaken roads as well over time.

Certainly, even paved paths that *never* have motor vehicles on them still need maintenance, and if that doesn't happen they'll deteriorate to the point of being unusable in a few decades even if nobody uses them.

So it's not reasonable to look only at the damage caused by specific vehicles, as that's not the only damage done to roads. It would also be reasonable to look at damage caused by Mother Nature (by whatever mechanism) and divide that up among the users of the path if we're worrying about which groups should be paying for a path's maintenance.