We've already talked about lofty concepts such as power & status quo.
To decide if this sign is the speaker, we need to talk about a couple of other lofty concepts: agency & consciousness.
Agency is the ability to act on your own choices. Speakers need to have some degree of agency, even if it is constrained.
The man walking the plank on the pirate ship has agency even if his options are limited.
But the sign has no agency because it is just a sheet of metal; it has no awareness and it makes no choices.
The sign is persuasive, but the speaker is the authority that put the sign there (probably a local council or the organisation that owns and maintains The Pipe).
For example, we could talk about the City Council as a speaker in this situation. We'd just have to also recognise that there may not be a single individual at the council who is the speaker for this issue; instead a group of people have coordinated to produce a single persuasive position on the issue, which in this case is, "People need to stay off the pipe!"