state highway vs superhighway

state highway and superhighway both are nouns.

Word	Noun	Adjective	Verb	Adverb
state highway	Yes	No	No	No
superhighway	Yes	No	No	No

As nouns, state highway and superhighway are synonyms defined as:

state highway and superhighway: a broad highway designed for high-speed traffic

Other synonyms of state highway include expressway, freeway, motorway, pike, throughway, thruway.

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