Buzzkill devices
The person who posted this image was utterly confused as to why city planners would put pizza cutters near staircases. The OP only noticed them after accidentally bumping his knee against one, which, as he explained, cause excruciating pain. Yeah, we can see why.
It turns out that these aren’t actually pizza cutters. They’re prevention devices to deter skateboarders from grinding down the edges. Yeah, as if that’ll work. If anything, these devices will force skateboarders to be more creative in how they grind on the ledge. Also, there’s a perfectly good rail to slide down on. Better luck next time, city planners.