interstate highway vs main road

interstate highway and main road both are nouns.

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

As nouns, main road is a hypernym of interstate highway; that is, main road is a word with a broader meaning than interstate highway:

interstate highway: one of the system of highways linking major cities in the 48 contiguous states of the United States
main road: a major road for any form of motor transport

Other hypernyms of interstate highway include highway.

interstate highway (noun)	main road (noun)
one of the system of highways linking major cities in the 48 contiguous states of the United States	a major road for any form of motor transport