45.—(1) ( a ) There shall be a speed limit of thirty miles per hour in respect of all public roads in built-up areas for all mechanically propelled vehicles.

( b ) The Minister may, with respect to the limit specified in paragraph ( a ) of this subsection, by regulations—

(i) increase or reduce it,

(ii) restrict it to particular periods of the day and night.

(2) Each of the following roads shall be a public road in a built-up area for the purposes of this section:

( a ) a public road which is in a county or other borough, an urban district or a town, other than a road which is declared by the Minister by regulations not to be a public road in a built-up area for those purposes;

( b ) a public road which is not in a county or other borough, an urban district or a town and which is declared by the Minister by regulations to be a public road in a built-up area for those purposes.

(3) The Minister may by regulations except any class of mechanically propelled vehicles from the built-up area speed limit.

(4) In this Act "built-up area speed limit" means the speed limit under this section.