The best I've ever found is Oregon state highway 31 near Silver Lake (dry lake), between Bend and Lakeview. There are two stretches that are arrow-straight and pancake-smooth and level. One is eleven miles long, the other is sixteen miles long.
It is totally out in the middle of nowhere, with no obstructions whatsoever to visibility, not even a tumbleweed. Anything bigger than a mouse, you will see it. Any other vehicles you can see coming for a couple of days. I've only been there once, but I figured out how fast my GS1100 could go there with soft saddlebags, a tent & sleeping bag bungee-corded to the back seat. With two teeth off the rear sprocket it was equal in fourth and fifth gear; maxed out at 600 rpm into redline in fourth, 500 rpm short of redline in fifth. I just went along in a tuck maxed out for a couple miles in each gear. And when I got tired of it and shut off, I still had about 8 miles to go before the road turned.
