One custom on two lane undivided roads is that vehicles move over into the breakdown lane so that faster traffic can pass. If someone is approaching you from behind, you move over, they pass you, and then they sometimes flash their hazard lights to say "Thank you". (You can, at your option, flash your high beams 'You're welcome', but though this is understood, it is rare.)
This convention is observed sufficiently regularly that failure to do so is considered rude. If you see two cars side-by-side coming towards you and the car which is overtaking has drifted slightly into your lane, you are also expected to move over into your breakdown lane. Failure to do so will probably cause a collision since neither of the two other drivers has much room to maneuver, and your combined speed is likely in excess of 220 km/h.
It occurs to me that for this custom to work a) the breakdown lane must be reliably passable at highway speeds, and b) the breakdown lane must generally be clear. (Well, and c) everyone has to know and follow the convention.) Both of these conditions seem to apply in the Western Cape. All the hard shoulders I saw were clear of sand or dirt, and in as good repair as the travel lanes. There were very few cars, people, or other obstructions on the shoulder. When I did see vehicles stopped they were pulled partly off the road and had their hazard lights on. It's likely this is because people know to loiter on the shoulder invites being struck at 120 km/h.
I don't know if this custom is observed anywhere else, but it is very useful here, as most of South Africa's roads are two-lane, undivided, hard surface, with wide shoulders. It means that you can pretty much drive at whatever speed you like, because if you are going faster than someone ahead of you, they get out of the way. If you want to drive more slowly than other people, you won't slow anyone else down, as long as you're willing to move over.