It would be easy to put a app on a phone that would make it not work while in motion.
It would also be just as easy to pass a law that says you can not use a cell phone in a car and make the fines very stiff.
The problem is the first one is business suicide. No one is going to buy a phone that does not work in a car.
The second one would be political suicide. No elected official is going to make a law that restricts cell phone use in a car.
They made a law here that says no texting while driving. Cops started writing tickets and when it got to court driver says I was not texting; I was on facebook.
Took them a few months to re-word the law to include multi media.