My first thought is that it potentially could have a warning system stating whether the system guaged its ability to drive as "safe", "be careful", or "hey I can't do this, you should drive." A noise or color system could be used to alert the driver. Perhaps on a freeway with little traffic that the Cruise has seen plenty of times, it could be fully safe. But on some backroad that it hasn't seen before, or during bad weather conditions, it would go into fully manual mode.
Spitballing here, but let's imagine a common impaired driving situation- being very sleepy and needing to get somewhere on a time-schedule.
I don't know if you've ever been in that situation, but it's possible to fall all the way asleep while driving. It's such a common thing that they carve rumble strips into the sides of highways.
Being so sleepy is a tricky thing. You can fall asleep even with the radio going full blast. I can't imagine any alarm system that would stop someone being lulled to sleep by the engine and movement of a car doing 90% of the driving.
