> Well, is one of many reasons why. I assume there are lots of edge cases. Floating plastic bags, for example.

A friend once almost swerved dangerously to avoid some scary-looking pipe pieces on the middle of the freeway. While contemplating making his move he saw a few more pieces fall off the back of a truck further ahead and bounce - PVC. He correctly calculated that swerving at high speed in traffic was more dangerous than just hitting a lightweight piece of PVC pipe.