I think I know how
The default password for many matrix signs is on the internet.
http://gadgets.boingboing.net/2009/01/23/default-password-for.html
Police intervention was required earlier this week when "political protesters" hacked a Sydney traffic sign and declared "Kevin Rudd sucks" to passing motorists in the city's Rose Bay. The declaration proved such a "major distraction" to passing motorists that cops were eventually obliged to gain access to the offending device …
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I agree, and unless Australians everywhere have spent the last few decades engaged in a coordinated campaign of global mis-information regarding the Oz national character, I think pollies (is that right) enjoy no more respect up there than they do down here.
But I can see that giving *every* driver on the road a really good laugh might be, er, distracting, and so there might be a road safety requirement to black out the sign.
These types of roadsigns are not remotely configured; they have a local PC control interface in that big yellow box at the back. Rarely locked, so it's said.. not that I'd know in person.
Here's more information for the curious (Inc. passwords and reset methodology.)
http://gizmodo.com/5136970/hacking-road-signs-is-frightningly-easy-and-funny-and-illegal
Many Oz signs have slots for the police to put in the bad-behaviour du jour.
E.g. Police Targeting: Speeding/Seatbelts/alcohol etc.
I suppose this is supposed to terrify you into submission but I take it to mean that you can speed if they are targeting seatbelts or drive drunk if speed is being monitored.
You could just snag the letters from your local church noticeboard and slide them in - I'm getting the letters for "Koalas" and "Rare Birds" because that makes more sense the the usual signs.