Maybe they could take some ideas from the rail industry, which have been dealing with this issue for quite some time now.

Engineers are required to to make some kind of input to verify that they're still alert, or the train stops[1] if they haven't done anything in X seconds.

Obviously you wouldn't want the car to just stop in the middle of the road, but there could be something like that designed.

[1] https://www.quora.com/If-a-train-driver-falls-asleep-what-ha...