Probably why the same drivers are so inattentive, those speeds on the highway would put anyone to sleep.
As far as the numbers go...my drive to Narromine: about 750 km, if I do 85 and save money, I will spend almost 9 hours of my life fighting to stay awake, at 100, still around 7.5 hours and still fighting, if I bump that up to 140-150 ( a nice comfortable speed on the open road), I'm down to a little more than 5 hrs on the road cruising alone in the :"zone", and at worst, I will have spent $30 more in fuel and gained a few more hours of quality life. I consider that a fair exchange.
NOTE: I'm not saying I do that, but I'd certainly like to be allowed to.