It is very hard to draw up fair laws to deal with accidents that are "caused" by factors like someone having drunk alcohol, smoked pot, using a phone, reaching for a mint, tuning the radio, trying to read road signs, being short of sleep, or just having a sexual fantasy.
Take the bloke who got a hefty prison sentence because he reached for a mint while driving alone on a motorway. While his attention was distracted (probably a second or two) he swerved, and eventually hit another vehicle and caused a crash in which deaths occurred. The court took the view that his inattention caused the crash, and the crash caused the deaths, so his attempt to get another mint caused the deaths. In short, manslaughter or whatever. But thousands of people do similar things every day without causing accidents (or not so obviously and undeniably) and they do not get punished at all - although what they did was exactly as culpable. No more and no less. Indeed, if the man in question had simply chosen not to mention the mint and his momentary distraction, he would almost certainly have got off without a prison sentence - perhaps without any punishment, or even blame, at all. He could simply have said "I dunno what happened - one moment I was driving along, then the car suddenly swerved..."
Or take alcohol. Once an issue reaches the public eye, and therefore the political limelight, there is an unfortunate tendency for it to blot out everything else - including common sense. A blanket ban on alcohol would be neither practicable nor reasonable. I recall an episode of "The Bill" in which inspector Gina has a few one night, drives to work the following day, is involved in an accident, and found to be driving under the influence. The factor that is always unmentioned in these discussions is "how long does it take alcohol to be completely metabolised" - because no one can say precisely. So a demand for "zero blood alcohol" means that, if you ever drink alcohol in any quantities, you must never drive a vehicle; and vice-versa. I don't think that would work, in a society where most of us (have to) drive, and most of us also like to have a few drinks.
Personally, I feel that while I prefer to be stone cold sober, I can manage a car safely after perhaps one pint of beer or a glass of wine. Two pints, a couple of glasses of wine, or any spirits, and I would call a taxi or whatever. This way, I think I stay within the law; but, more important, while my driving is certainly impaired, it is within acceptable limits.
How can I say any impairment is acceptable? Simply by comparison with other everyday impairments (some of them unavoidable), such as: having a cold, having had an argument, being angry with my boss, having had less than 8 hours of sleep every night for the last week, or even driving through a cloud of toxic fumes emitted by a bus or lorry. Or listening to the radio, or having two passengers conducting a conversation in the car while I'm driving. Or worrying about income tax, or my pension, or a leaking roof... Not to mention musing happily about some sexy film star, or thinking about my next holiday. Or, as has been demonstrated by experiment, being distracted by trying to make sense of a forest of road signs. The list is long.
Actually, I would like to see much more attention given to the role of sleep deprivation in causing accidents - and not only on the roads. Our culture actively conspires to deprive people of the sleep they need to be at their best, by forcing many of us to work 40-hours weeks (or longer), often with ever-lengthening commutes, and insidiously encouraging us to make the most of our leisure as well. Result: burning the candle at both ends. How many crashes are caused by drivers who never even realise they are falling asleep?