... then we need to do it rigorously.
Figure out the threshhold above which elevated risk becomes criminal (i.e. "it is illegal to drive in such a manner that you have more than X% chance of hitting someone else per mile/per minute"). This is a policy matter -- maybe it's okay to have up to double the normal risk of collision, but no more?
Then test the hell out of everything. Levels of drunkenness, of stonedness, of distraction (from "putting on makeup" to "in car with pretty girl/guy"). Being old. Being young. Being male/female/black/white/purple. Driving past flashy billboards. Driving through speed traps (speed traps cause wrecks, ban the things). Driving while tired ("nope, sorry, after your 14 hour day you can't drive; you're impaired, take a nap first").
That's really the only way to be fair with this sort of thing.
Or we could just treat people as responsible, and not worry with forms of impairment that people assume voluntarily and can do away with if they need to. Talking on the phone while driving is fine, so long as you're willing to say "In traffic now, have to go for safety."