At the driving school I attended, they had us accelerate to 50 mph and then slam on the brakes so we'd have a feel for the distance (and the feel).

While it's true they don't stop instantaneously at highway speeds, cars shouldn't be driving highway speeds when a pedestrian suddenly being in front of you is a realistic risk.

What if the obstacle is not a person? What if something falls off a truck in front of the vehicle? What if wildlife spontaneously decides to cross the road (a common occurrence where I live)?

I don't think these problems can just be assumed away.