I think you're making a bit of a strawman here that people usually do with respect to utilitarianism. The utilitarian answer with 5 people on an island, where 1 is a doctor, 3 need organs, the remaining person has available organs, and the 3 are necessary to keep the group alive -- is to kill the one person and take their organs.

However, in a society where people can observe the actions of others and form motivations in response to policies, etc, you'll find that because society reacts fairly poorly to organ harvesting, because organ harvesting is implausible to do at scale without extra bad things happenning, etc, the utilitarian solution is actually not to go about doing it.

Only a naive utilitarian wouldn't try to also remain consistent with something like a Kantian imperative of global self-coherence.

Now, as for cars and testing self-driving on real folks, well, this may be something where the water is pretty murky. I think that society will react poorly enough to early bad events in self-driving that a measured approach is actually the best for saving lives in the long term.

The critique here shouldn't be that "well, utilitarianism sure looks good from afar, but would you murder your neighbor?" It should be "The problem is too difficult to address with utilitarianism because it involves complex societal factors and responses."