are there plans to add in the 500 series county routes in New Jersey? they use a system that keeps the same number across county lines unlike the 600 and 700 routes which are unique only in their respective county
Maybe in the distant future, but many of us fear opening up a can of worms by including any system designated "county". I'd personally be against it even though I've got mileage on many of them.
I see both sides of the issue, though I'd add quite a bit of mileage were this system to be added. Each county maintains the routes, but NJDOT is nominally responsible for assigning the route numbers. (I say nominally because some NJ counties may disagree with NJDOT how to sign sign routes in the field, and the county puts up the signs.)
As to the can of worms: I think we reached a consensus that Virginia secondary roads would NOT get done even though the state maintains them, thanks to massive duplication of secondary route numbers. (Hypothetically, every county can have a route with the same SR number with no two so-numbered roads ever touching.) That's kind of the inverse of the NJ 500-series issue. Perhaps a more relevant question might be: are the "Winnipeg City Routes" (Winnipeg, Manitoba, Canada) (canmbw) actually maintained by the province, or by the city? I don't know, but perhaps someone on the forum does, and the answer could be relevant here.