It really annoys me when I have to constantly look at the speed to avoid getting above the limit and it is very tiring for long drives.
This is similar to the problem for pilots, who can be distracted by mundane tasks due the complexity of controls in modern aircraft. If these tasks are removed, the pilot can focus on what's more important.
According to NASA " For the most part, crews handle concurrent task demands efficiently, yet crew preoccupation with one task to the detriment of other tasks is one of the more common forms of error in the cockpit."
https://asrs.arc.nasa.gov/publications/directline/dl10_distr...
The moment the back of my mind doesn't have to handle precise throttle control I find my mind wandering and my spacial awareness is shot. I guess maintaining speed is the fidget spinner that keeps me focused on the task of driving.