Mr. Beepy.
Oh, wait...this is the real world.
Actually, I want to tie my bike to the roof rack somehow.
Let's tie Mr. Beepy to the bike rack! You can cut him down somewhere in Idaho. He'll never bother either of us ever again.
Right... while clothesline might work, I think you'll find better poly rope at the hardware store...
I'd think a 3/8" nylon rope would work well. It won't fray like a cotton rope would. Just try several of them on their rolls in the store to see if a knot will hold well. You might have to burn the cut ends so they don't unravel. Get enough to go around the bike several times, wrapping around the roof rack and the center bar of the bike each time around. You might also tie it at least once, front to back. That way, it won't shift around up there when you're doing full speed on the highway.