One of the problems that I see with raising the speed limits in the US is (from my limitied experience some 14 years ago in KY) the lack of (serious) driving school and how easy it is to obtain a license.
The second would be the (afaik) lack of mandatory regular technical inspection of the vehicles.
Maybe you do have that (in some states perhaps) - but I am not aware that it is a general requirement - here you have to go and get such a technical inspection (which covers basically all safety aspects of a vehicle - such as brakes, tires, suspension, structural integrity etc. as well as a tight regulation on parts (rims, steering wheels, lights, etc.)) every two years for about any road going vehicle.
And finally the third: I might be wrong again but I think there is no special speed limit and mandatory resting periods for trucks. Here trucks have a speed limit of 50mph and the times a drive may drive continuously is also regulated. After a certain amount of driving a certain period of rest is required. Driving times as well as speed are recorded at all times. This I think is extremely important for general safety.