Many years ago, in France, in the olden days before satnav (never mind satnav with vehicle dimensions) we got into the wrong lane to cross a river and were heading for the tunnel rather than the bridge. We hit the hanging plastic strips that are there to warn you of exactly that mistake. It was a hell of a bang, we changed lanes but couldn't stop to examine the damage - we expected the front to be totally caved in. But - not even a mark. They are clearly really light and just make a big noise, but we got enough of a fright to make us a lot more aware now.