This road sign is owned by the UN and is legal administrator of it, its use has always been 'optional' by states parties. Its meaning is held in the "UN Convention on Road Traffic, Road Signs and Signals", and has a history dating back to the League of Nations circa 1922. Many countries teach their drivers about the sign, but don't necessarily use it domestically.
Sign: C,17(a) "END OF ALL LOCAL PROHIBITIONS IMPOSED ON MOVING VEHICLES". (Categories of driver, eg L, P platers, AND categories of vehicle can remain speed limited once on a derestricted road, eg; heavy trucks, caravans).
A sign C,17(b) is the "END SPEED-LIMIT" sign. It appears almost the same as the derestriction, but within the circle and its diagonal bar/s are numerals representing the ceasing speed-limit. With the C,17(b) a 'default' (typically rural) speed restriction will typically apply. (So each is similar but legally different, at law).
AS1742.4 of 1999 has the speed derestriction cataloged as an "R4-2 speed derestriction" sign, and its meaning (think - 'under another law of this jurisdiction' at ARR 25); "that no speed limit applies beyond the sign". This date edition of AS1742.4 complies with UN Convention, an edition circa 1984-1999 did not - it read 'rural default' applied. Not permitted under the legal contract so change was made to correct AS1742.4 to bring it inline with the rest of the world team. Even earlier editions did comply with the legal owner.
AS1742.4 Edition 2008 has simply removed ALL reference to the R4-2 speed derestriction sign. This is an attempt by leftist academia (Monash), and certain state road agency staffers who are each on the committee overseeing the AS; to prevent any future road agency, or elected government - by way of 'policy' from using speed derestriction signs once again.
So, C,17(a) is our R4-2 (rem not existing in the current AS1742.4).
C,17(b) is our R4-12, EXCEPT our End Speed Limit sign hasthe english word, eg; "END 50", and is not symbolic. (Dumb really, Australian road safety 'experts' have no place designing road signage). BUT- the intent between each is much the same; that a rural default generally applies, and the AUS unique design version 'R4-12' sign can only be used where a rural default is so regulated.
NT CLP are reintroducing speed derestriction to certain highways. I am proposing derestriction signage, with the plate "DRIVE TO CONDITIONS" at NT border entry points, in similar fashion to this NSW example.http://www.ntnews.com.au/article/2011/08/12/253241_ntnews.html