eteb3
@mundik, sorry I missed your request for the tagging convention. 

my understanding is that the access tags refer to legal access, not practical access - and there is no indication that motor vehicles are legally not allowed here.

The problem your driver identified is that that the surface and gradient makes access practically impossible for a van. 

4wd_only=yes is a tag that was developed in Australia and is used in a few places elsewhere. I've also added the tags surface=unpaved and gradient=75%. This way the access=* tags capture the legal status, and the other tags can be read by routing software. Hopefully those routers will route elsewhere for ordinary vehicles, with 4x4/4WD vehicle routing as a special option. 

eg, Graphopper will route a car up Grass Hill (wrongly, in my view), but will not route a van or lorry there (play around with the transport types and see) 
https://graphhopper.com/maps/?point=The%20Warren%2C%20RG4%207DY%2C%20Reading%2C%20United%20Kingdom&point=St%20Peters%20Avenue%2C%20RG4%207EY%2C%20Reading%2C%20United%20Kingdom&locale=en-GB&elevation=true&profile=car&use_miles=false&selected_detail=Elevation&layer=Omniscale

See also https://wiki.openstreetmap.org/wiki/Key:4wd_only

Hope this helps. @Russ McD I'll leave this open for @mundik for a while, then close. 

