Ignorance is not, and never has been, a defence. As a licensed driver you are responsible for knowing the rules of the road.
And if you can't slow to 40kph without notice, you would therefore not be able to stop without notice. This would be a contravention of the basic tenet requiring all drivers to drive safely to the conditions. I.e. you would be considered 'driving in a manner dangerous'.
Oh, and as someone who has often worked at the side of the road with lights on and drivers speeding past, these laws are not before time. SA has had a requirement to slow to 25kph past emergency vehicles for many years - the sky has not fallen.