Pure utilitarianism leads to outcomes that are clearly out of step with almost everyone's moral codes. For example you could kill someone and take their organs to save the lives of 4-5 people. Is it rational that we're not allowed to do that? Why do some people get to keep 2 kidneys when there are others with none?
This is solved at least somewhat by using 'rule utilitarianism' instead of 'act utilitarianism'. Society is better off as a whole if we adhere to rules such as protection of the human body or safety regulations when constructing buildings.
The question, as always, is: "Where do we draw the line?".
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.
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?
No, but that should be the ideal which you should strive to move towards, when practicable. But you can't ever actually get there, and shouldn't try infinitely hard either.
Killing someone to take their organs to save 5 lives is utilitarianism to only first order effects. We live in a world that does not terminate after one time step, so we have to consider Nth order effects to calculate utility.
For example, the second order effect is other humans' moral judgements. "How horrible, he murdered that man" is a valid moral reaction to have, and this is disutility that must be accounted for in a "pure utilitarian" world view. Third order effects may be the social disorder that results from allowing such ad hoc killings as means to an end, and so on.
The only thing preventing pure utilitarianism from being viable is a lack of compute power, and "rule utilitarianism" is a poor heuristic based approach for philosophers without patience ;)
There was a pretty good short story, "Dibs" by Brian Plante, about that published in the April 2004 issue of Analog Science Fiction and Fact.
Everyone was required to be registered in the transplant matching system. Occasionally you'd receive a letter telling you that someone was going to die soon unless they got one of your organs. That person now had dibs on that organ, and if anything happened to you before they died they got it.
Usually you would get another letter a couple weeks or so later telling you that the person no longer had dibs, which generally meant that they had died.
Sometimes, though, you'd get a second dibs letter while you already had one organ under dibs.
And sometimes you'd get a dibs letter when you already had two organs under dibs...meaning if you died now it would save three lives. At that point you were required to report in and your organs were taken to save those three other lives.
The story concerned someone who worked for the transplant matching agency who got a second dibs letter and was quite worried. He illegally used his insider access to find out who the people were who had dibs on him, and started digging around to try to convince himself they were terrible people who didn't deserve to survive to justify illegally interfering, if I recall correctly (I only read the story once, when it was in the current issue).
I don't remember what happened after that. I just remember thinking that it was an interesting story and explored some interesting issues.
Reliably the people absorbing the risks capture almost none of the added value.
Yes, it's a great idea. Killing one person will allow us to double speed. How exactly I wonder, but this seems like the kind of project where asking questions is strictly forbidden.