These concrete barriers have been put up by either the RTA or Council to stop cars from colliding head on at corners like this - impatient drivers put their foot down way too much, and I have seen one end up on the footpath at this spot.
Given that it's such a dangerous spot for cars, what sort of idiots would run out onto the road and spray graffiti on the barriers? These were painted a few weeks back, and already tags are reappearing.
I am intrigued though as to why these barriers attract graffiti. This is an area of high net worth - a house around this spot will cost you a million dollars or more. There are no schools nearby, there are no skate ramps. The Bay Run is not an obvious attractant for teenagers with something to prove. I can understand graffiti appearing near skate ramps, or under freeways (which are great place for teenagers to hang out and be naughty), but on a wide open stretch of the Bay Run, which is really only popular with people over 30 who are walking, running or ricing for fitness? It just doesn't fit.