(a) There shall be a speed limit of thirty miles per hour in respect of all public roads (other than motorways) 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 council of a county or the corporation of a county borough in bye-laws under section 46 of this Act 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 council of a county in bye-laws under section 46 of this Act to be a public road in a built-up area fo