you would enforce this in an age when almost everyone uses GPS on their phone to figure out how to get where they are going in their car.

You would almost need to regulate automakers and force them to make a phone mount standard equipment on all new cars. Or maybe force cellphone manufacturers to include the mount with every new phone they sell. Otherwise, I don't know how practical it is to try to force hundreds of millions of drivers to buy, install, and use these mounts.