they have to make a lower speed limit on inferior roads. I think the speed limit on I-15 should be 85-90.

Is that the speed I would recommend for all drivers/cars/conditions? Of course not. It is where I would set the maximum legal speed. There are a lot of drivers who routinely drive 80 MPH. I have a UHP neighbor who says he won't pull over people if they are driving under 80 (unless there is some other reason). It is a safe speed under most conditions. The 65 MPH "limit" is a farce and it leads to farcical speed limits on every other major road.