Do you believe we should make the national speed limit 25? If not, you're accepting that people will die needlessly, and that the value of a human life is not, in fact, infinite.
Consider if by sacrificing someone on an altar you could magically cause several months of construction work to happen overnight. That would still be murder.
The current traffic system as an insane "death and mayhem lottery" that we force ourselves to play, with out respect of youth, age or anything.
The current interest and action towards bike-friendly cities is a symptom, I think, of a healing of societies' psyches. We have been pretty brutal to each other since the Younger Dryas, and it's only recently that we've started to calm down and think about what we really want our civilization to be like.
The real thing I would advocate (if this weren't a beautiful Sunday afternoon, calling me from my keyboard) is a design for traffic that began from the premise of three (or four) interconnected but separate networks, one each for pedestrians, bikes, and rail, and maybe one network of specialized freeways for trucks and buses. Personal cars would be a luxury (unless you live in the country) that few would need (rather than a cornerstone of our economy) with rentals taking up the slack for vacations and such.
But if you're interested in this sort of thing, don't bother with my blathering, go read Christopher Alexander.
My other answer is really just an invitation to a kind of thought experiment: what if we really did restrict ourselves to just walking, biking, and trains? How would civilization look in that alternate reality?
If decreasing your speed from 100km/h to 50km/h gives you a 1% lower chance of dying in a road traffic accident, but you spend an additional 2% of your life stuck in traffic, is that a win?
They may be more like classic cars than regular vehicles at some point, though.
I think people underestimate the cultural impact that self-driving vehicles will have - imagine a whole generation or two after self-driving vehicles are generally available - how many people will bother learning to drive? I think it might become more of a job-specific skill than a general 'adult life' skill as it is now in most places.
At some point, human driven cars become novelties, just like that. There is no reason to ban them, but as you suggest, maybe there will be some HOV-like lanes where they don't really have access. Or even some time constraints (not during rush hour on some key roads, not in lower manhattan, etc).
You could have picked a better example. I see tons of young people smoke and wonder what the hell is wrong with them, whether or not they haven't been paying attention for the last 40 years and then I realize they didn't because they weren't there to begin with. So the tobacco industry can work their nasty charms on them with abandon because there are new potential suckers born every day.
Or alternatively, maybe as strange as a motorcycle or vintage MG.