To your question: The question that the parent comment tries to answer is "what do we do so the backup driver does not watch his phone?". This is a solved prob

To your question: The question that the parent comment tries to answer is "what do we do so the backup driver does not watch his phone?". This is a solved problem: you fit the experimental car with an interior camera aimed at the driver. If the backup driver is observed to be distracted by the interior camera, the car can sound a warning for the driver to be alert and if they ignore the warning, the car's hazard lights would come on and it would be slowly brought to a halt.