The court will never require the officer to provide documentary evidence that the sign was "erected or maintained by lawful authority of a public body or official". The court can simply take judicial notice of this fact pursuant to Evidence code §452(h), which states:
"Judicial notice may be taken of the following matters …
(h) Facts and propositions that are not reasonably subject to dispute and are capable of immediate and accurate determination by resort to sources of reasonably indisputable accuracy."
If the defendant feels the sign does not follow the proper guidelines, it is up to the defendant to prove why the sign is improper; this is part of his defense and is his responsibility.