I've read about this before. And driven (way to fast) in Montana and Utah (which have an 80 mph limit in many places).
It isn't just the speed limit that keeps driver's speeds down. People seem to decide that "this is fast enough" and drive at the speed they like. Even I settle into a cruise that isn't all that fast considering the car has a 255 kph limit in the computer and is rock solid at well over 200 kph. (I haven't tried to push the limit on this one (yet) )
What I found was that the traffic was moving at a little over the 80 mph and there were few going slower than the surrounding traffic and very few going significantly faster. I've read that this is not because of the legal limits but because of what people are comfortable with.
In Montana, as I recall, this was just around the 80 mph and up the middle of Utah (on 3 occasions through there) it was around 90 mph. The lighter traffic and very wide highway in Utah may account for that.
In some circumstances it is possible that traffic moving at a more uniform speed even if a little higher nets out to safer than traffic with 20 or so mph difference in speeds.