>How do you counter a utilitarian argument without humanizing the situation?

Have you considered that perhaps there is no argument to be countered? That your entire foundation of "humanizing" the situation is foolish and off base? Alternatively, you yourself blithely dismissed the fact that your "humanizing" easily works both ways, in fact more easily works the other way: many of us have had friends or loved ones killed by driver error, tiredness, drunkeness/distraction, or in simple accidents, deaths that would not have happened if a computer had been in control. I don't need any "what ifs" because one of my friends was killed by a drunk driver right near her school. Those millions who are going to die have friends and loved ones too you know. It is in fact a statistics game, and one of economic tradeoffs. That's human life, in fact life in general.

>no amount of statistics would convince me that the tradeoff was worth it

Which is why we do not leave societal decisions to you, but instead have a system that attempts (with varying success in some cases) to put significant distance between the moment-to-moment emotions of specific individuals and enactment of law governing all. You may not care about this imaginary tradeoff of your loved one vs thousands of other people's loved ones, but those other thousands of people certainly would. So you can and should be outvoted.

>Who bears responsibility in that case?

It would depend. If it was just an honest mistake that could not have been reasonably caught, then the answer is "no one" and it'd just be covered by insurance, or even the government, so you could say that "society itself chooses to take responsibility for the greater good". We already have a model for this in many, many other scenarios because the basic issue here is very much not a new one. A big, successful example would be vaccines, where we all have to take on a small risk for a very great general good and there is government provided compensation in the rare case that something goes wrong. However as far as individuals go, sometimes there is in fact no one "at fault" or responsible. It's human to always want something to blame but part of maturing is accepting that freak risk is a fact of existence.

>I suspect that ordinary people won't accept that for a long time to come.

We literally do accept it, and have accepted it for all of recorded history. Much of mythology comes out of the basic recognition that there are forces beyond humanity.

>At least with a human in the mix, we're all still operating within the same social and legal order.

Debatable, but also beside the point and it doesn't change the objective fact that a human in the mix is a proven recipe for failure. Your philosophy is cold comfort in the face of tens of millions of deaths. You are also ignoring the large percentage of deaths that are purely down to things such as animals crossing the road, or poor driving conditions that are not reacted to appropriately, or where a driver kills themselves (and perhaps passengers) due to human failure but involves no 3rd parties, which automation could also positively effect.

You're also ignoring positive utility, which is a critical component to the economic and societal equation just as the car itself is. Self-driving cars change the dynamics for city space usage, for people who are unable to drive due to age or medical conditions, for the poor who cannot afford the capital outlay of a full car, for those who are discriminated against by human drivers, for the environment (long charging times for electric vehicles become significantly less of an issue when the vehicles can simply go take care of it themselves while you're doing something else), and on and on. It's hard to overstate how central this is to society up and down the stack. Your viewing lens is entirely too narrow, which is precisely why individual emotional response is a terrible basis for governance.