I've read some articles about testing top speeds on Salt flats.
EDIT:
Yep, they're in Utah somewhere. Search google for Bonneville Salt Flats, and you'll get this great little discription:
The Bonneville Salt Flats
Imagine a place so flat you seem to see the curvature of the planet, so barren not even the simplest life forms can exist. Imagine the passing thunder of strange vehicles hurtling by on a vast dazzling white plain. This is not an alien world far from earth; it is Utah's famous Bonneville Salt Flats.
The Bonneville Salt Flats is one of the most unique natural features in Utah. Stretching over 30,000 acres, the Bonneville Salt Flats is a fragile resource administered by the Bureau of Land Management.
Thousands of visitors, commercial filmmakers, and of course, high speed auto racers, make the Bonneville Salt Flats a world famous destination. We hope you enjoy your visit to this natural wonder on Utah's public land.
http://www.utah.com/playgrounds/bonneville_salt.htm