Sorry Duggie but I don't necessarily agree with you on that one.
Most of these roads are in speed derestricted areas so the state limit applies. That doesn't mean they are rated to any specific speed. The prevailing conditions of the road would change far too regularly for that. Also, I'm pretty certain sections of the GRR are actually signposted 80ks and I think even less at some river crossings.
Regardless of all that, yes it may be better to drive at a speed that makes corrugations easier to traverse, but when approaching another vehicle, common sense says you should slow down. I get it that trucks find that difficult, but smaller vehicles have no excuses.
What happened to the OP and others here in these situations is a disgrace....