superhighway vs main road

superhighway and main road both are nouns.

Word	Noun	Adjective	Verb	Adverb
superhighway	Yes	No	No	No
main road	Yes	No	No	No

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

Other hypernyms of superhighway include highway.

superhighway (noun)	main road (noun)
a broad highway designed for high-speed traffic	a major road for any form of motor transport
an extensive electronic network (such as the internet) used for the rapid transfer of sound and video and graphics in digital form