Every discussion I've read on this issue ignores the fact that there could be passengers in the car, and there's no reason to disable their phones. Very much the opposite, in fact, because they can be looking up addresses, getting directions, etc.
if the trip starts at Junior's high school, and mom and dad's phones are at work, the driver has been identified — Junior is driving.
What if all three phones are in the car? Which one does it disable then?