How fast is safe is very subjective. And many people will claim they can drive a speed safely but in reality they don't or can't.
IMO, speed limits are established at a safe speed for the majority of people in a majority of situations.
As individuals, we have a choice: drive at or just under the speed limit; drive faster and risk a ticket; drive slower and risk a ticket.
Personally, I have no problem driving 55 or 65 or 75 as is posted. I don't feel a need to go 10+ miles over the limit just because I can.
I see more people having to slow down and getting frustrated because traffic isn't doing "their preferred speed" than I see people driving "too slow".