Tthere is no strong correlation (well, there is correlation but it is not a causative relationship) between speed and safety per se. The safest speed is the general speed of traffic.
I don't believe an 18 wheeler doing 80 mph is safe. Especially when you get passed by a convoy of 6 all doing the same speed. There just comes a point where you have to back off because its just not safe to stay up with this type of crowd.
I live near I44 and once it turns wet it becomes common fare to end up with a jack knifed or overturned 18 wheeler. Maybe the 4 wheelers are the cause but when you are driving a rig as big, you have to consider that you aren't going to be the best handling vehicle if you need to make a sudden move. That's a lot why I normally stay around 60 when towing.
AAA of Missouri does an annual survey of its membership and usually within the top 4 items of concern is the speed of the 18 wheelers on Missouri Interstates.
Bottom line if you get involved with an 18 wheeler in some type of accident, you lose.