Verges and hedges are trimmed by mashing machines (they don't so much cut as mangle and fling across the road) that have one single 'arm' that extends over the item to be cut. They can't cut through metal posts that road signs are on, so the arm is lifted up and put down behind the post. This leaves a big bit around the sign that hasn't been trimmed. Someone is supposed to come along behind with smaller trimming equipment (weed whacker, strimmer, hand shears whatever...) but of course, this doesn't always happen so the signs get obscured.

Mind you... the councils are not above putting a sign warning of a new roundabout a) less than 30ft from the roundabout it was warning of and b) behind a tree.

LTS