The productivity or pleasure of driving can also be negative. An easy way to find out would be to ask people how much money or time they would pay (or take) for having their driving time extended by each minute or mile.
As people who speed, do so willingly, they always have a subjective gain there.
Any reasonable speed limit compromise between risks and time should apparently depend on the quality of the cars and the roads and the value of time. As both of these increase, the speed limit should have considerably grown since the 70s...