Most states have default statutory speed limits for cities and highways, but these are over ridden if the road has speed limits posted. Sometimes an artificially low speed limit is posted for "revenue enhancement" purposes. One time when I was summoned for jury duty in traffic court I annoyed the prosecutor a bit during voir dire. He asked me what I thought about the speed limit on a stretch of road near my home and I replied that after driving it every day for over a decade, I thought it was deliberately set too low for just that reason - revenue enhancement, and after all, our city WAS on the national speed trap list.
The prosecutor then said "Sir, what if I told you that per the law, the posted limits are to be regarded as reasonable and proper?"
I replied, "Counselor, if the legislature felt a need to define something as reasonable and proper under the law, I'd say that's pretty darn convincing evidence that they know it's UNreasonable and IMproper in fact!"
He didn't like that answer - and liked it even less when the rest of the jury pool laughed out loud, and even the judge had to suppress a snicker.
He didn't argue, he just moved to the next juror. (I didn't get picked as a juror for that trial.)