Someone called?
Several interesting issues raised here and I will try my best to give a semi-legal response as a non lawyer. This advice is ONLY applicable for WA as it relates to the drafting of the Road Traffic Code 20000
Firstly, local councils do not have the legal authority to install shared path signage, any that they do provide are unlawful but, here is the irony, the way the legislation is drafted the path would become a shared path anyway.
The only body legally allowed to install regulatory shared path signs or path markings is Main Roads WA but, apart from PSPs, MRWA do not sign shared paths. So unless the council erects formal regulatory signs, unlawfully, most paths are not signed.
To be a shared path a sign is not necessarily required. The definition of a shared path also includes "any path where one of its main uses is for cycling". As a result if anything suggests it may have cycling as one of its main uses then it probably is.
In the case of the footbridge this has a PBN bike route sign pointing along it, this effectively confirms the "one of its main uses for cycling" test. Also the presence of centreline would imply cycling as a main use, so again it is reasonable to presume it is an (unsigned) shared path. If its wide and looks like a shared path then I, personally, would not have a problem riding on it.
One caveat to this, if it isn't signed as a shared path and it is not unreasonable to presume it is one, it would be prudent to cycle slowly keeping speeds to about 10 kph. Seems very slow but, for short distances, not unreasonable when sharing a footpath.
Finally, while it is unlawful for councils to erect formal regulatory shared paths signs, it would be OK to provide informal signs which would then confirm a legal shared path.
Probably the greatest irony is regulation 216. This creates the offense of riding on a footpath
216.Shared paths and separated footpaths
(1)The rider of a bicycle who is 12 years of age or older shall not ride on a footpath, that is not a shared path or a separated footpath.
But, at the same time it effectively confirms that one of the main uses for footpaths is for cycling (albeit by under 12s). Untested in law but yet another possible interpretation by which a footpath could be considered shared.
Clear? Not!