Bar10Dah: Your nice little table assumes a single driver who is able to maintain that speed safely -- e.g., on a rural interstate with not much traffic. If you think about the whole highway full of drivers, things change. On congested highways, if everyone goes 70-75 whenever they are able, there will be a lot of accidents as they come up on stalled traffic.
I used to travel from Worcester into Boston for an 11 am appointment. It would take 1 hour on the Mass Turnpike (an interstate divided hiway). It would take 1 hour 10 minutes on a route with less traffic (I290 to I495 to Rte 2 for locals). Eventually I always took the slower route because there were lots of accidents on the Mass Turnpike closer to Boston as cars going 60 to 70 caught up to stopped traffic. That 1 hour on the Mass Pike would sometimes become 2 hours or more, especially in the winter.
My theory is that traffic goes too fast causing more accidents that then slow down traffic. If everyone went a bit slower, on the average everyone would get there faster.
--Walter