As you can see, the walkway is completely covered by chain link fence - even directly overhead. What I'm wondering is how they got the sign on the OUTSIDE of the fence.
The only way I can think of is they climbed onto the fence at the edge of the walkway and sort of rock climbed across the walkway until they got to the center. That's plain crazy.
Whoever did that should be locked up in a psycho ward as a clear danger to himself and others should he have fallen off and onto a passing car.