They take into consideration type of road [possibly speed limits] but they forget about the "human factor." That's the one where we need to eat, sleep and go to the bathroom. The need to top up with Gas also appears to have been overlooked and then we get on to traffic, construction delays etc. With a cross country drive on Interstate they seem to think you can drive at the speed limit from the moment you back off your driveway to when you pull up at your destination, impossible.
As a time tested rule here at RTA it is thought that an average of 57mph in the West and 55mph in the East on main Interstate/highways can be sustained with all these factors taken into account .[other than sleep of course]