nometallic road

[carbonman]

I was out and about yesterday, and I came across a sign pointing up what was virtually a green lane, but the sign said, "nonmetallic road".
I later checked on my OS map but its not listed there as anything other than what looks like a normal b road...
Obviously I was forced to drive up it and check it out which proved good as it lead to some nice woods trails....

Does anyone know anything about these?
Are there likely to be more of these lurking around providing another opportunity for off road fun!
Cheers
CM