Suppose a short stretch of road has a 25mph speed limit, but 99% of motorists drive 35mph there. Would it be proper for the government to install some hidden cameras, wait a year, and then send out thousands of citations for all the times people had driven 35mph there in the year since the cameras were installed?
I would argue that it would not. Regardless of any notice offered by the sign, I would argue that the government's clear and overt failure to enforce the speed limit over that one year period would constitute effective notice that it had no interest in doing so and that renewing enforcement of the speed limit would require some new notice of intention to do so.
None of these principles is offended in Jefferson's case. While it is true that congressional representatives are not always held accountable for taking bribes and many legislators probably think they have free reign to engage in such behavior because of lax enforcement, I can guarantee that the average reasonable person still believes bribery is illegal.
I agree entirely. I was not trying to argue that the failure to pursue such violations in the past should bar their pursuit in this case, but rather to point out what other condition would have to be met (the fact that the law in question must be widely ignored). I should perhaps clarify that it's not sufficient that the law be ignored by violators, but also that the law be ignored by those who happen to comply with it (i.e. such people must be indifferent to the fact that other people are breaking it).
In general, ignorance of the law is no excuse. There are some exceptions for when mistake of law is a defense to a violation, but I've never heard of nonenforcement being one of them.
As for your traffic example, I think the sign says it all. Assuming there is no statute of limitations problem, I don't see that there would be any due process violation. I agree it would be a pretty jerky thing to do, but the due process clause only requires the government to be fair. It doesn't have to be nice. I see nothing unfair about prosecuting violators in an evenhanded manner.