This is my pet peeve and something I see most days when I am on my motorcycle. It is not just young adults it is mature adults as well. It is dumb and should be banned.
If you put any value on the life of your children and the life of anyone else, then ban the use of cell phones in the car, whether you are driving or not.
Lead by example, do not use your phone in the car. If you need to use it stop to the side of the road and get out of the vehicle.
If your children are found using their phones in the car then take them away from them for a period of time. If they continue with the practice of using them in the car then take them away from them permanently.
Save their life & practice tough Love