The Expert will be specific knowledgeable people at the Department of Transport, Arrive Alive Communications, the Road Traffic Management Corporation or others who might have the expertise to answer the question.
If the question is about FREEWAYS, then the rule is that motorcycles of 50 cc or less are not allowed on freeways at any time of the day or night.
Here is the official wording from the Road Traffic Act:
323. Special provisions relating to freeways
(1) No person shall operate on a freeway—
a motor cycle having an engine with a cylinder capacity not exceeding 50 cubic centimetres or which is propelled by electrical power or which is a vehicle as contemplated in paragraph (b) of the definition of motor vehicle in the Act;
a motor tricycle or motor quadrucycle;
This rule is the general rule if there is no road sign governing motorcycles on freeways.
On the freeways in Jhb that there are signs that say ‘No 125cc’ motorcycles beyond this point." This is probably where the person got the idea from.
So ... the general rule is: NO M/cycles 50 cc or less on ANY freeway.
On some freeways, probably where there is a high density of traffic, there may be a further restriction on that stretch of the freeway that prohibits any M/cycles that have a higher engine capacity too ... like 125 cc. I have not seen the actual signs myself, so I am not in a position to state exactly what such a road sign is prohibiting.
As always, ANY rule of the road applies unless there is a road sign or a traffic office giving a different instruction in a particular situation.