Update: The statewide National Functional Classification (NFC) and National Highway System (NHS) reviews are planned to begin in December of 2013 if FHWA approval of the adjusted census urban boundaries is received before then.
 
Federal-aid highways are defined in Title 23 of the United States Code. There are two other federal-aid definitions - the specific federal or state program of interest should make clear which definition to use.

Federal-aid Highways, Federal-aid Systems & Federal-aid Eligible

The definitions in this area refer to arterials, collectors and local roads. These terms come from National Functional Classification (NFC), a transportation planning tool. All public roads are classified according to the function they serve within the overall roadway network. In addition to defining federal-aid eligibility, NFC allows roads to be studied and compared across different regions of the state or the entire country.

Federal-aid Eligibility: Using NFC Maps 

Urban Areas and the Urban Statewide Review:

Update: The Proposed adjusted census urban boundaries have been submitted to FHWA for approval, once MDOT receives an answer, MDOT will contact the agencies by U.S. Mail.

The Michigan Post-2010 US Census Urban Statewide Review being coordinated by MDOT...