Long, but not important

[dgolub]

CR 501 in NJ is 53 miles long, but, where it doesn't overlap with NJ (and NY) 440, it only exists to access local communities from bigger roads. Unlike other long county routes, which traverse rural areas with an otherwise underdeveloped road network, CR 501 parallels I-287, then NJ 440, then I-95, then the Palisades Parkway and US 9W.

More broadly, some of the 500 series county routes in New Jersey are longer than any state route.  CR 519 is the longest one, and it runs entirely through the middle of nowhere in the western and northwestern areas of the state.

[ftballfan]

Michigan has M-57, which runs from the outskirts of Grand Rapids to the northeast suburbs of Flint, but only goes through one city with a population above 3,000 (Greenville) and goes through zero county seats.