In most places bicycles are forbidden to use Interstate and other limited access highways. The only places that allow bicycles on them are where alternative roads either don't exist or are considered so far away they are impractical. When you have to be on an Interstate it's because there is no good alternative.
Not always so. In a lot of western states, bicycle riding on Interstates anywhere outside of urban areas is perfectly legal. In Oregon, for example, I can legally ride the length of I-5 from near the junction of I-205 south of Portland all the way south to the California line (with the exception of a section through Medford). And I can ride 84 east of the Portland Metro area east to Idaho.
But I agree with everyone else who says try to ride on the Interstates as little as possible.