Georgia instituted a law this year

It's your basic "hands-free" law. I still see a ton of folks on my commute with their phone in their lap, and the driver staring down at it, looking up at the road every few seconds. You are correct that it is basically impossible to enforce.

On the other hand, it does change some behaviors. I drive older cars that are not pre-wired for bluetooth connections, so I went out an bought adapters, that allow me to answer calls hands-free. I probably wouldn't have done that if the law hadn't been passed. So I suppose the law is making some marginal impact.