That's also a solved problem - Mercedes cars have "attention assist" which can figure out how attentive you are by measuring how you drive. If you are literally falling asleep at the wheel, the system will get very annoying and insist that you stop for some rest.

Of course, whatever algorithm they are using won't work if the car is driving itself, you can't judge the reaction time if all the driver is doing is holding the wheel.

They've been tweaking it pretty regularly since the beginning. The initial release had no timed events at all, and only asked the driver for hands on the wheel when the system's confidence dropped below a certain level. Then they added some timers as well (I think before this crash) and have been tweaking the situations and durations. Right now, I believe at highway speeds it's either 1 or 3 minutes depending on whether you're following a car or not. (The system has more confidence when it has another car to track, so it's more lenient there.)