Never had an accident at high speed? I should think so, buddy, because if you did then you wouldn't be here to brag about the fact. And yes, your speeding IS my business because YOU, sir, are endangering the lives of my family.
Only if the speed is excessive. This has nothing to do with the speed limit which is fundamentally arbitrary. Different vehicles, drivers and road conditions should all be factors. Another thing that is never taken into account is speed-related attentiveness. A speed of 40kph might incur a lack of attentiveness such that a pedestrian will get hit at that speed, where if the driver had been doing perhaps 80kph they would have been paying more attention and actually come to a stop or near stop before impact.
As for the open roads like those out in the Karoo, there should be no speed limit. All the accidents on the black spots on the N1 occur because the road is monotonous and people are not travelling fast enough to maintain attention. They either need to drop the limit or rebuild the road with curves in it to keep drivers alert.
Motorcyclists should definitely be subject to different speed limits. No-one pays attention to the road and other vehicles like a rider, especially if they're travelling 200kph or more. In accident motorcycles on a motorway a motorcyclist is likely to kill or injure only themselves, they have greater manoeuvrability and they can bring their vehicle to halt far quicker than any normal car.
It has been well-established by research overseas that the majority of people gravitate to a speed that feels right and comfortable for them, their vehicle and the road, regardless of what the signs might tell them is the 'right' speed. If we insist on ignoring this basic fact we can only expect that people will ignore the speed limits.