Yes, the people likely to be put in these decisions have already been prepared, prepared to the point where they aren't even making moral decisions anymore. Doctors doing mass-casualty triage aren't going to be debating ethics, they're just going to follow accepted process. Society does not expect untrained people to make these decisions either, as most of us would either freeze or run around trying to save everyone, even if it's logically impossible. When there was no time left to think, we'd just go on instinct. This is, of course, a descriptive analysis.

If you take the other approach and try to derive a proscribed solution, what we should do, then it becomes an analysis of underlying values. I suppose this is the whole point of the exercise. Do you value 5 lives over 1, decisive action over passive acceptance? Who should benefit from your moral choices: people, animals, society? A person that valued all life equally could justifiably argue that saving 5 dogs warrants killing 1 person, though probably not in a human law court. If 2 reasonable people come to different conclusions, then there's some underlying difference in values that accounts for this. An Afghan warrior and a Wall-St. businessman are probably going to come to significantly different conclusions. Of course, I'd say both were horribly wrong