Calling a US# highway 'secondary' doesn't quite sound right. Sure, it's not an Interstate, but
https wrote:
The secondary road system within a county for the purpose of this subchapter consists of those roads maintained by the NCDOT that do not carry "N.C." or "U.S." numbers and are outside the boundary of any incorporated municipality.
(note - in NC nearly all roads, with the exception of those in municipalities, are maintained by the state. In most other states, counties are responsible for secondary roads.)
This Iowa county even calls it's road department the 'Secondary Roads Department'.
http://www.johnson-county.com/dept_sec_roads.aspx?id=1371