4.  The provisions of article 3 above shall not apply—

(a) in respect of a vehicle being used for police, fire brigade or ambulance purposes;

(b) to anything done with the permission or at the direction of a police constable in uniform or of a traffic warden;

(c) so as to render it unlawful to cause or permit the whole or any part of a vehicle to remain stationary in the boxed area if, when the whole or any part of the vehicle entered that area, the person so causing or permitting had reason to believe that he would immediately be able to cause or permit the vehicle to be driven out of that area.