'Traffic free' is a better description (even if some cyclists on here want to identify themselves as 'primus inter pares' traffic) than 'off road' which in the cycling community and externally is interpreted as off metalled surfaces. The key here is
who you're trying to communicate with: it's normal people without an agenda and not commentators like
@mjray and
@Tim Hall (and
@Spinney has added a thoughtful opinion, though I appreciate the point they're making but think that your work to scope minimal but useful lighting on such routes is not the place for that argument), as you said:
Everyone understands what's meant by 'traffic free' in the context of cycle paths/tracks: it's probably metalled, maybe gravel, or rougher, but there will be no vehicles on it. 'Vehicle free'?