Let's say that the car is entering a fog or other weather condition, which it realizes is beginning to obstruct its visibility or control (e.g. snow, heavy rain, wind). After visibility or control impairment has crossed a warning threshold, but before putting safety at risk, the car can alert the driver and begin the process of handing over manual control.
I think the idea is that at Level 4, the car can drive the way humans drive. If it's unsafe for the car to drive autonomously, it's unsafe for humans, too.
