Different classes of traffic may be prescribed by a scheme under section 16 above in relation to different parts of the special road to which the scheme relates.
The classes of traffic prescribed by any such scheme shall be prescribed by reference to the classes set out in Schedule 4 to this Act.
The Minister of Transport may by order amend the said Schedule 4 by varying the composition of any class of traffic specified therein or adding a further class of traffic to those so specified, . . .