Bird-brained railway bridge - Seaford - 15.9.2014

The inner faces, i.e. on the road, have been given smooth plating and the tops, as can be seen above, have been formed into quite sharp ridges. So why, if all that thought has been given to stopping human misbehaviour and mishaps, has this multi-storey pigeon perch been created?
The line underneath is the single track between Seaford, to the right and Bishopstone to the left.