I've ridden a fair amount of Interstate miles- they'd definetly be a lot more fun without all the traffic and exhaust.

And I've pulled 50 miles per hour off the North Cascades Highway without drafting (heck, even in Seattle proper there's hills you can break 50MPH on a bicycle- Innes Arden Way past ShorelineCC for the locals-)

doing 50MPH on a bicycle you've got to trust the rubber and take the lane!