Roe posited 'viability' for reasons that are similar to those you've raised in your vehicle example (though I'd quibble about whether killing a pedestrian the way you describe is vehicular manslaughter - normally 'gross negligence' must be proven beyond a reasonable doubt, or breaking some other law, e.g., driving while texting or while intoxicated).

The extreme 'pro-Choice' position is not that a woman has a right to an abortion after her water has broken; rather, a woman has an absolute right to 'remove' any person (adult or fetus) that is present in her body without her consent. The right of a fetus to remain in the womb is no stronger or more special than the right of any adult who wanted to enter (or stay after they were no longer welcome). Seen that way, a woman does not have a 'right' to an abortion when the fetus is viable, but rather, she has a right to self-defense.

Which is bizarre. Every American is well aware of the laws about guns, ownership, and rights to self-defense from, say, a home invasion. About 100 Americans die during home invasions each year.

Meanwhile, about 700 women die in childbirth each year. If a woman feels that she is threatened, why is her right to defend her body less than what we give to people defending their homes?

I am not entirely sure I am a pro-Choice extremist. The 'right to privacy' is so fundamental in America (every technology, medicine, and economic relationship as we know it emerged largely because a right to privacy made it possible to transact and trade on that privacy). Rather than the maximal 'self-defense' approach, the 'right to privacy' approach leaves some space for nuance. But if the right to privacy fails, then what remains other than self-defense? (The benevolence of legislatures? except...if a Texan travels to California to murder another Texan, whether California permits that murder or not, the Texan is still a murderer - this is not something that can be left to state legislatures/federalism...)