Depends. If there aren't any other through roads in that area, then it is legal to take the interstate on a bicycle. In some places you'll see signs saying "bicycles must exit", or "bicycles prohibited". I assume if I don't see one of the latter signs when I get on the freeway and haven't passed one of the former, that it's allowed. I'm not aware of a master map or site showing where bikes are and aren't allowed on the interstates. I figure the worst that happens is that you get dressed down by a state trooper if you happen to transgress and then have to find a way around whichever segment it isn't allowed on. It's possible they'll give you a ticket, but very unlikely. Don't take what they say at full face value though, very often police don't actually know the laws and do what they want, leaving things for lawyers to sort out later.