Thanks a lot for the warm welcome, support and resteem!
I agree that in reality it's impossible (especially due to our inability to foresee the future, as you pointed out well) and unwise to determine the value of a human life, but in some hypothetical cases we would simply have to make some decisions that would show what human life we consciously or unconsciously consider more valuable, and that's intriguing to me both as a philosophical and as a psychological topic.
"In the car example, you could have offered a third option: drive off the road and be killed yourself, but you didn't." - Wow, nice note, that would be an interesting addition! (and now I wonder: maybe I'm not altruistic enough to even consider that option, and that's why I haven't remembered to include it :/)
One fun fact about the unintentional damage done by inventors to millions of people: Ethan Zuckerman, the man who discovered pop-up advertisement, has publicly apologized a number of times for his annoying discovery. : )
"We're like pebbles dropped into a pond. There will be ripples that radiate outward from our landing point that will disappear over the horizon of our life's time span." - Nicely put! I think that your (hopeful) butterfly effect is something that can leave a positive impact on the society, go for it!
Cheers!