FWIW, in most (all?) states, the only mandatory speed limits are those printed in black on a white sign. Many states use black on orange for construction zone limits, but those are only advisory (same thing with black on yellow). Some states have wised up and started to use the mandatory black on white signs in their constructions zones, I dunno about NJ. Even in those cases, it may not be a legal speed limit - in many states there are baseline speeds defined for different types of areas (like 30mph in residential areas with less than 100 yards between houses, 35 in industrial areas, etc) and any posted limit that varies from the "standard" for the type of area requires a speed study by the state DoT. Many, many municipalities just put up speed limit signs and skip the speed study for money, convenience or even ignorance of the law. In many cases, having an illegally posted limit negates the whole shebang leaving you with no actual limit.
Unless the state laws have been written to provide for mandatory construction zone speed limits, then posting a black on white speed limit sign could have the opposite effect - removing any speed limit at all.
And while in such a case, there may be no specific limit, you may still get ticketed for reckless driving or somesuch. But, if you were just "keeping up with traffic" such a charge may not stick.
I'm no lawyer, and I've never been ticketed for speeding, not even warned. The above is all stuff I wandered across when I was bored one day.