In Kansas, some highways have signs specifically restricting them to faster-moving traffic. I take K10 a lot, and at every entrance there's a sign that says that pedestrians, bicycles, motorized bicycles, horse-drawn vehicles, and other slow-moving traffic is prohibited.
I don't know about the main interstates, though. Even if you could ride on them, would you really want to? That'd be pretty much suicidal. You really ought to look into mapping programs or even spend some time attempting to figure out back-road routes on Google Earth. The back roads are prettier anyway, and there's less traffic and somewhat lower speeds.