interstate has 1 distinct synonym spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is intrastate.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
one of the system of highways linking major cities in the 48 contiguous states of the United States
involving and relating to the mutual relations of states especially of the United States
“Interstate Highway Commission”
Synonyms, antonyms and the broader and narrower relations for interstate come from Princeton University’s WordNet. Full sources and licences.