From OpenStreetMap Wiki
|National Highway System|
|Used on these elements|
|Tools for this tag|
Several countries have a defined National Highway System network that is independent of numbered routes. The tag is currently used in the United States and Canada.
- NHS=yes - normal value to indicate that the road is on the NHS
- NHS=STRAHNET, NHS=STRAHNET (connector), NHS=Interstate - other values that are in common use in the U.S. to describe subsets of the NHS