You're correct in that states where weed is legalized are still struggling to adapt to the DUI situation, since no equivalent of the breathalyzer or the one drink/hour guideline exists yet. But domestic violence from stoners is pretty much unheard of - biggest argument might be over who ate the last of the doritos. And unlike alcohol you really can't OD on it. Marijuana needs to come off federal Schedule I so that research can be performed - right now medical research is effectively illegal.