No person shall operate on a public road a motor vehicle on which is displayed a licence disc or a licence and roadworthy certificate disc, whichever the case may be, or anything purporting to be a licence disc or licence and roadworthy certificate disc-
(a) which is not applicable to such motor vehicle;
(b) which is in any way obscured or has become illegible, except if such licence disc or licence and roadworthy certificate disc, whichever the case may be, is temporarily obscured or illegible by reason of a cause beyond the control of the driver of such motor vehicle.

