Make a law that requires cars to broadcast a blocking signal that makes cell phone and texting signals blocked any time the engine is running? Or anytime the wheels are turning, allowing for maybe the heater or accessories to run when stopped? Or a combination of the two. or ???
Today one TV show "expert"(don't remember his credentials) mentioned that talking on a cell phone while driving is equal to about the same as being drunk (odds of an accident) and that texting is about double that. Even if he is way off on the exact numbers there is still a problem.
The only thing I can see as a big problem, is that it would be easy to shut it off without being caught. Maybe signal strength meters measuring the blocking signal transmitted from your car used by cops just like radar guns would help enforcement.
Or spot inspections, just like inspection sticker checking stations temporarily on the road.
It might be difficult to keep a blocking signal from spreading to another car that is stopped and using a phone, right next to you. Maybe some variation on a Faraday cage ??????????