interstate highway vs interstate

interstate highway and interstate both are nouns.

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

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

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

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