state highway vs throughway

state highway and throughway both are nouns.

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

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

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

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

state highway (noun)	throughway (noun)
a broad highway designed for high-speed traffic	a broad highway designed for high-speed traffic