interstate vs interstate highway

interstate and interstate highway both are nouns.

Word	Noun	Adjective	Verb	Adverb
interstate	Yes	Yes	No	No
interstate highway	Yes	No	No	No

As nouns, interstate and interstate highway are synonyms defined as:

interstate and interstate highway: one of the system of highways linking major cities in the 48 contiguous states of the United States

interstate (noun)	interstate highway (noun)
one of the system of highways linking major cities in the 48 contiguous states of the United States	one of the system of highways linking major cities in the 48 contiguous states of the United States

interstate (adjective)	interstate highway (adjective)
involving and relating to the mutual relations of states especially of the United States