that if you can't see it

Though they'd look a lot better with big buckets over their heads if their instructions to keep left were to be temporarily ignored, buckets would probably be quickly thieved, thoven or possibly thifted. Lightly wrapping something over the arrow would only really have an effect if you'd never seen such a sign before and needed the arrow to know which side of the item one was supposed to remain. Given road-modification crews' skill in seemingly easily demolishing much more robust structures such as water mains is odd that the signs were modified so delicately when a quick semi-accidental swipe with the beak of a mini-excavator could simply and permanently remove them from their island.