I am a pensioner who is not on the road at these peek times so my response is probably unwarranted.
However, I do travel a great deal between Johannesburg and Durban and usually am on the road in the early hours of the morning. I do not know the statistics for crashes involving heavy vehicles between 5am and 7am, BUT what frightens me is the number of times I have had to take evasive action, especially between the towns Villiers and Warden on the N3, due to truck drivers apparently going to sleep at the wheel.