That is precisely why there is no need for a separate law. Intent is already considered. And it doesn't matter if you were looking at the stereo or looking at the cellphone, if you were too distracted to drive, then one law can cover that. And it doesn't matter if you were violent because you were drunk or violent because you're just violent all the time — that issue is covered by differing sentencing guidelines for repeat offenders. One law, covering multiple situations.
Well, that's not really how it works. Violence is a crime that requires intent and there is little room for interpretation. Being distracted isn't a crime by default and there are perfectly legitimate reasons why you might find yourself distracted. But there are some things that are so distractive and risky that it's simply best to outlaw them alltogether like cellphones and drink-driving.
I know people who can drive and talk on the cellphone at the same time successfully. I know people who can't drive worth a shit, period.
That's just an (unconvincing) anecdote. Almost every credible study out there shows that all people have much worse control when using a cellphone, which is hardly surprising.
FWIW here's my personal experience: I know people who drive while on the phone and people who drive shit. There seems to be a strong correlation between "driving like shit" and "driving while on the phone". It's most likely an Illusory superiority that leads to both.
The only law of that sort which might make sense would be to have a driving test which tests your ability to drive while talking on the phone, but we're not even close to that.
That's like testing a jet pilot by by having them take off and land once, or judging a bodyguard by looking beefed-up and walking behind someone. The situations where it really matters are so far out of the ordinary that you can't reasonably exclude that someone won't be a risk.