Definitionally, L3 is not ready for the human to stop paying attention. I would argue that you need at least L4 automation before you can explicitly advise user

Definitionally, L3 is not ready for the human to stop paying attention. I would argue that you need at least L4 automation before you can explicitly advise users that they can do distracting things in the car, and even then only in good weather and on specific roads (e.g. highways).