Aren't the yellow corner speed signs just advisory anyway? ie not legally enforceable. At least in NSW they are.
I don't think there is a single place. Roads are maintained at a state and local level, and the data isn't usually freely available
But your VMS has to be taking its speeds from somewhere. Where and what is that database and who maintains it? Mostly probably there is one, but its not available for public consumption, or at least requires a fee.
Then the next question is how accurate is this data and how often is it refreshed? My TomTom also posts the speed limits as it thinks they are, but sometimes is wrong. Seems silly that management are relying on data that might be out of date.
Try these:
I believe open street map has a maxspeed attribute -but the data is crowdsourced again so don't know about the accuracy.
http://www.openstreetmap.org/Search for the road and you'll see the data in the left pane- Max speed is what you are after. Not all road show this though.
Or if its a section of road out of town, its a bit harder. Zoom to the section of the road in question. Then click Export (it will want to save it as an osm.file., change that to XML). Open the file and search (Ctrl& F) for maxspeed
eg: the line you want is Tag v = 110k k=maxspeed
<tag v="trunk" k="highway"/>
<tag v="110" k="maxspeed"/>
<tag v="Great Northern Highway" k="name"/>
or
tag v="secondary" k="highway"/>
<tag v="110" k="maxspeed"/>
<tag v="Broome-Cape Leveque Road" k="name"/>
or try this:
http://www.speedalert.com.au/products.phpOr this: You could use Google streetview and go back until you see a sign.
Or this:
http://www.wikispeedia.org/ - crowdsourced - poor data for Australia