ring road vs main road

ring road and main road both are nouns.

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

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

ring road: a highway that encircles an urban area so that traffic does not have to pass through the center
main road: a major road for any form of motor transport

Other hypernyms of ring road include highway.

ring road (noun)	main road (noun)
a highway that encircles an urban area so that traffic does not have to pass through the center	a major road for any form of motor transport