I don't think the ethical questions are that difficult really.
This 90% isn't an end game position. This is a trolley problem in a scenario where the track continues and an endless sequence of trolley problems with multiple switching positions that, if managed well, will get us to as close to zero road deaths as we can ever possibly get.
The question isn't "is it good enough?" when the alternative that we currently have is to leave those 90% to still die. In the balance of "good enough" questions, "is it good enough to sacrifice 9 people because we couldn't save the 10th?" should be the absolute loser every single time. I can't imagine why you're arguing the case against that.
Where do we set the limit for robots? It's hard to answer that question without first establishing the parameters of regulatory control, liabilities and responsibility, standardised safety protocols, information sharing protocols and so on. But if 90% doesn't fit into that limit against our current piss poor performance, we have definitely fucked up. Honestly, I might balk at 5% or 10% improvement. Largely on the grounds that much of it may be very difficult to accurately attribute. At around 15%, I personally start feeling comfortable pushing the yes button.
In my mind, the question should be; is this good enough for now? That's because, as stated, this isn't an end position. Achieving that 90% more doesn't mean you stop, it means you can have more resources to work on eliminating that other 10%. You can't actually work on that remaining 10% if you haven't dealt with the that starting 90% in the first place.