[OSM-newbies] Tagging "Unsuitable for motors"

[Paul Warren]

Is there a convention for tagging UK roads signed as "Unsuitable for  
motors"?  These are roads that are technically a legal right of way  
for cars, but are deemed unsuitable for normal cars and are typically  
not maintained by the authorities.

highway=unsurfaced seems the closest, but such roads may actually be  
surfaced and are deemed unsuitable for other reasons.

It seems to me that this is a place where OSM can do better than  
other maps which typically mark these roads as normal unclassified  
highways, despite it being a fairly important distinction for someone  
trying to navigate using the map.

Paul