What Constitutes Distracted Driving?

A while back, Farhad Manjoo suggested an alternative to anti-texting while driving laws:

A better system wouldn’t make distinctions about what we do on our gadgets, but would instead look at the effects of our actions. The best rule would simply say, Don’t do anything in your car that could be unsafe. In 2009, Maine adopted just such a policy. Its law doesn’t make any particular technology illegal in the car. Instead, it bans “distracted driving”—driving while you’re engaged in any task that could impair you.

Balko rejects this idea:

[Y]ou’ve just taken the problem of too many jurisdictions having varying definitions of distracted and made it exponentially worse. Now whether or a motorist has violated the law is at the individual discretion of every police officer in the country. You now have about 600,000 different definitions of distraction. Some studies have shown that having kids in the back seat is actually more distracting then a .08 BAC or talking on a cell phone. That in theory could now be illegal.