I think that the capacity of a road stays the same regardless of the speed limit. Yes, it take a car less time to do a certain stretch of road, but the distances between cars are greater. By how much? Let's assume everyone drives using the 3 second (or 2 second, or whatever) rule regardless of speed. That means that for each lane one vehicle passes a certain point every 3 seconds. This doesn't depend on the speed of the vehicles (above a certain speed, of course).
Aspera: Your example of the two drivers, one driving for 5 hours and the other 8h, reminds me of a local politician who wanted to win the votes of both drivers and ecologists by proposing to get rid of all speed limits. He argued that the faster you drive the less fuel you need, because you have less time to use fuel, etc.
Of course, driver fatigue is very important, but I don't believe that driving at 80mph for 5h is safer than @50mph for 8h.
In most European countries (with some local exeptions) the speed limits are:
- 50 km/h urban,
- 90 km/h rural,
- 110 km/h expressways,
- 130 km/h highways.
During last year's oil price hike some countries lowered the last two to 100km/h and 120km/h, resp. Since then more countries are thinking about doing this (I believe that CO2 emissions has something to do with it). Unfortunately, at the same time Poland's ministry of transportation proposed
raising those speed limits to 120km/h and 140km/h. Why? Nobody knows...