Driving a motor vehicle takes 100% concentration, 100% of the time. The fact that after an extended period of any activity focus decrease, is not in dispute. As Michael states, there are very good reasons why professionals are restricted in time and distance they may cover in any given 24 hour period. Few drivers on the road are better drivers than the professionals. And those who are, have the sense to limit their activity to maintain optimum focus.

Lifey