there is some discussion in https://community.openstreetmap.org/t/help-needed-review-of-road-access-restrictions-mostly-massachusetts/131168

> Lastly, if a “No Trespassing” sign is posted by a private individual, does that alone justify tagging a road as access=private? 

yes, if it applies to road (note that some people post intentionally misleading ones)

> particularly when services like mail or trash collection still occur.

these can occur on access=private ways