I confess I was completely ignorant of the point, because I've only ever had to deal with it in the context of the relevant statutes here in Victoria: Road Safety Act 1986 s 3 (definitions of highway, road and road related area) and section 3(2)(a) (declaration of roads), and nowadays, Road Management Act 2004 s 11.
When I re-read those provisions, I see that the power to declare a road is discretionary, so that there could still well be a role for the common law about dedicating roads. Frankly, I expect nearly any criminal case we care to think about disputing "road" would be decided on a factual basis about whether or not the area was open to the public. But, it could possibly crop up in cases such as those involving roads not open to the public, like on Defence bases or other areas not open to the public. (I mention that example, because after many years pondering I'm still yet to figure out how civil police reckon they can book folks for speeding or driving when disqualified on defense bases, given they're not open to the general public. The Commonwealth Places (Application of Laws) Act 1970 (Cth) can apply either state Road Rules, or the Commonwealth version — National Transport Commission (Road Transport Legislation — Australian Road Rules) Regulations 2006 — can apply, but any way you look at it, road still has an "open to the public" element in the definition.)
Evans J set out the common law test for dedication of a road as a highway:
 Both dedication by the owner of land for its use as a highway and use by the public of that land as a highway must occur to create a highway otherwise than by statute, Cubitt v Lady Caroline Maxse (1873) LR 8 CP 704 at 714 and 715 and A-G v Biphosphated Guano Co (1879) 11 Ch D 327 at 340. An intention to dedicate a proposed highway may be insufficient in itself to constitute a dedication if the proposal is abandoned before the highway is built: Healey v Corporation of Batley (1875) LR 19 Eq 375 at 385 to 387 and Tottenham Urban Council v Rowley  2 Ch 633 at 642 and 643.
 It is convenient to go to the law as enunciated in Halsbury's Laws of England, 2nd ed, Vol 16, 1935 for relevant common law principles on the dedication of land as a highway. This edition of Halsbury was current in 1944. The following passages, with citations omitted, are taken from it:
"212 A 'highway' is a way over which all members of the public are entitled to pass and repass; and, conversely, every piece of land which is subject to such public right of passage is a highway or part of a highway. ...
258 Land dedicated by a person legally competent to do so to the public for the purposes of passage becomes a highway when accepted for such purposes by the public; but whether in any particular case there has been a dedication and acceptance is a question of fact and not of law.
259 Dedication necessarily presupposes an intention to dedicate – there must be animus dedicandi. The intention may be openly expressed in words or writing, but, as a rule, it is a matter of inference; and it is for a Court or jury to say whether such intention is to be inferred from the evidence as to the acts and behaviour of the landowner when viewed in the light of all the surrounding circumstances.
260 Acceptance by the public requires no formal act of adoption by any persons or authority, but is to be inferred from public user of the way in question. Even if an express intention to dedicate is proved, it is necessary to prove also that the way has been in fact thrown open to the public and used by them.
The evidence from which Courts or juries are asked to infer both dedication and acceptance is, as a rule, open and unobstructed user by the public for a substantial time.
261 An intention to dedicate land as a highway can only be inferred against a person who was at the material time in a position to make an effective dedication – that is, as a rule, a person who is absolute owner in fee simple and sui juris. When, however, a primâ facie case is proved of an intention to dedicate, express or implied, it lies upon the defendant to show that the state of the title to the land is or was such as to render any such intention inoperative.
271 [T]here is no fixed minimum period [of user] which must be proved in order to justify an inference of dedication, and no fixed maximum period which compels such an inference."
Declarations made under the Road Management Act 2004 aren't declarations for the purpose of the Road Safety Act 1986. I can find many gazette declarations under the former, but not the latter — mind you, that doesn't mean they're not there, just I can't readily find any — and that also might be a reason why the common law could come into play, perhaps say on new estates under construction and not yet officially opened to the public.