18.—(1) Subject to paragraph (2), every sign shown in a diagram whose number is indicated in column (2) of an item in Schedule 17 shall be illuminated in the manner and at the times specified in column (3) of that item.
(2) Where a sign shown in a diagram whose number is indicated in column (2) of an item in Schedule 17 is placed for the purpose of conveying to vehicular traffic a warning, information, prohibition, restriction or requirement which applies only at certain times, the sign need only be illuminated, and in the case of the sign shown in diagram 776 shall only be illuminated, in accordance with that Schedule at those times.
(3) Where a sign shown in a diagram whose number is indicated in column (2) of an item in Schedule 17 is illuminated by a means of external lighting, then that means of lighting—
(a)shall be fitted to the sign or the structure on which the sign is mounted or which is otherwise specially provided; or
(b)if the sign is mounted on a bridge, tunnel or similar structure over a road or is provided temporarily at road works, the means of lighting may be mounted in any other manner such as to illuminate the face of the sign effectively.