No.
Could your knowledge of the speed limit be wrong? Could the speed limit have been set at some other amount? Is it theoretically possible that the local government could repeal the speed limit altogether?
In other words unless you are arguing that speeding is absolutely wrong your point fails to compel me.
That's the problem with analogies...they never work perfectly. However, in response to:
Could the speed limit have been set at some other amount? Is it theoretically possible that the local government could repeal the speed limit altogether?
Yes. The local government could repeal the speed limit altogether. However, dropping the analogy and moving on to the topic directly, ignorance of an absolute morality is not an excuse of being judged by it.
In regards to the government. If such a thing as absolute morality exists...and the government does something that allows for the violation of that morality, am I then freed from that standard just because the government says so?
Or should I still adhere to that standard despite what the government says I am allowed to do?