Yes, INTERSTATE is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, T 1, E 1, R 1, S 1, T 1, A 1, T 1, E 1.

Is it a valid word?

INTERSTATE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: interstate highway.