> Would you rather be hit by a semi-truck going 1mph or a bike going 28mph?

I'm actually not sure. The initial impact would hurt more with the bike, suggesting that perhaps momentum or something else proportional to velocity (not v^2) is a good heuristic there.

But if I tripped after impact, the bike would do no further damage to me and the semi would be ~1s away from killing me.

The semi-truck will come to a stop a lot slower than a bike, and because I won't be able to avoid either I'd rather get hit by the bike.