|the highway functional classification of a road in the United States|
|Used on these elements|
|Status: de facto|
|Tools for this tag|
This key indicates the highway functional classification of a road in the United States as designated by a state department of transportation according to guidelines established by the Federal Highway Administration (FHWA).
Highway functional classification is a well-established practice in the traffic engineering field in the U.S. Each state's department of transportation is responsible for managing a Highway Functional Classification System (HFCS) for all roads within the state with a high level of consistency.
The FHWA's criteria for functional classification incorporate many of the same considerations that factor into highway=* tagging in OpenStreetMap, such as the level of access control and connectivity. For this reason, newcomers to OSM are often tempted to apply HFCS to highway=* very literally, contradicting even basic global definitions of that key's values.  Unfortunately, individual HFCS classifications do not straightforwardly translate into highway=* values, partly because the two schemes make different distinctions and partly because they serve different purposes.  Nevertheless, HFCS is of interest to many mappers and potentially a data consumer that prefers OSM to official datasets. Relegating an HFCS classification to the HFCS=* key allows highway=* to better reflect on-the-ground conditions in many situations.
Globally, designation=* is the normal way to record an official legal designation. However, a given road in the U.S. may have multiple designations from various state and local agencies, which sometimes use the same terms with different definitions, making it necessary to store each designation in a separate key.
How to map
To map an HFCS classification, you would need to consult the state department of transportation's official logs or map of classifications. Note that many state governments retain copyright over state-authored documents and maps, even if a state open records law requires them to be made publicly accessible.  Avoid copying from a copyrighted work unless you know for sure that OSM has permission to use it.
Set HFCS=* to a value corresponding to an official HFCS classification in Title Case with spaces between each word, for example Urban Minor Arterial. Set source:HFCS=* to the source you consulted for the classification.
As of 2021, no editor, quality assurance tool, or data consumer offers specialized support for this key.
The original TIGER 2005 import chose highway=* values based on CFCCs, an alternative functional classification system formerly used by the Census Bureau. It quickly became apparent that these classifications were suboptimal for a general-purpose map, overemphasizing route membership, so mappers began making piecemeal, ad-hoc exceptions according to differing personal criteria. Some mappers turned to state functional classification maps as a guide, while others coalesced around a set of criteria based on a mixture of function and physical characteristics.
HFCS=* was first introduced in June 2010 on about 16,000 ways. Since then, it has steadily increased in usage as the originally tagged ways have been split many times.
- Highway Functional Classification Concepts, Criteria and Procedures – Federal Highway Administration