ignorance not bliss

It is said that ignorance of the law is no defense. I don't know if that is always true legally; it may be. But, if it is true in an absolute sense, it is unjust. (For those who remember the subjunctive tense, this is not it, because I don't know whether or not it is true.)

Eg., suppose the speed limit is reduced from 55 mph to 25 mph with no signs to let people know. I would consider it unjust to give drivers traffic tickets for not slowing down when there is no way they could know that the speed limit had changed. Also, if a sign were posted, but hidden by bushes, or blacked out by vandals, drivers should be ticketed (assuming, of course, that they were not the ones responsible for the sign not being legible.)

Any absolute statement should give pause for thought on whether it really makes sense.