Years ago I walked into one that was at the height of my right kneecap and ever since then I have taken an appropriate amount of caution around them while walking. A few years ago I punctured my bumper cover with one and it didn't do any damage to the pickup. So we both drove off after taking a look. Totally my fault.
I patched it up with some vinyl contact paper from Micheals and it didn't look half bad. Could not tell from a distance or at night that there was a hole there. My main concern was to not have an entry point for debris.
Having said that I would prefer that people would remove them if they are not being used.