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:

Urban Areas (UA) are released after each decennial US Census as Urbanized (UZA), population greater than 50,000 and Urban Clusters (UC), population between 2,500 - 49,999, both UZA and UC are collectively called urban areas. Title 23 of the USC Chapter 1 - Federal-Aid Highways Sec 101(a) (36) Urban area - following text paraphrased - allows MDOT and appropriate local officials to cooperate and adjust outward the urban boundary of those census urban areas with a population greater than 5,000, subject to aproval by FHWA. Once UA's are adjusted and FHWA approved, they are called Adjusted Census Urban Boundaries(ACUB).