Hmmm ...
"Also, in the light of concern about vehicular use in the countryside, new provisions were introduced in the Countryside and Rights of Way Act 2000, so that TROs can now also be used for the purposes of conserving natural beauty, including flora, fauna and geological and physiographical features. It is for local highway authorities to decide if, how, and when to introduce a traffic regulation order on a right of way.
The Government recognises that motor sports enthusiasts driving in the countryside often do so in a responsible manner; the TRO legislation is only aimed at illegal, anti-social and environmentally damaging use. Defra guidance highlights the need for all the interested parties to work together to consider the various options where there is an identified problem."
As you say, it's up to the local Highways authority .. which means we must all do our bit by checking local TRO's and complaining about them as and when, but preferably before, they happen!