Is it a case of determining the likely outcomes?
At most speeds my braking distance is governed not by the braking distance, but by the likelihood of something ahead throwing up stones.
At speed, a small stone would probably be deflected over the car by the air stream. To enter the passenger space, something would have to be quite heavy, and already dropping practically vertically at quite a speed. If that was the case, the hood might stand up to stones, but not rocks. But the probability of that happening with exactly the right timing and trajectory is not very high.
No doubt I'll be now hit by an engine falling off an aeroplane.
