A couple things come to mind.
One - you can't effectively ban driving distractions. People get distracted by all sorts of things, including thoughts. I love the fact that I can talk to people while I'm in my car, hands free on my phone through my car speakers and built-in mic. I find it less distracting then when there's someone else physically in the car talking to me. So, should we ban passengers in cars? Spouses, children, friends, dates? I'm guessing not many will think so.
Two - we don't need to rely on laws to keep people focused. You can always be an active participant in society. For example, honk at them. A honk and friendly wave might bring them back to the task at hand. Sure, they might get all mad and flip you off, but at least you tried. That wouldn't be easy for me to do, as I tend to take social anxiety to a whole new level, but I'd like to think that I'd get in the game if I thought that lives were at stake. Plus, if you're willing to honk at someone because of a one-second pause after a light turns green, you should be willing to honk at them if they're potentially going to get in an accident and cause you a lot more than one second.