This from 2001.
The U.S., with its 214 million motor vehicles, has paved 3.9 million miles of roads, enough to circle the earth at the equator 157 times. In addition to roads, cars require parking space. Imagine a parking lot for 214 million cars and trucks. If that is too difficult, try visualizing a parking lot for 1,000 cars and then imagine what 214,000 of these would look like.
However we visualize it, the U.S. area devoted to roads and parking lots covers an estimated 61,000 square miles, an expanse approaching the size of the 51.9 million acres that U.S. farmers planted in wheat last year.
[Note: 640 acres in a square mile. 61,000 square miles times 640 equals 39,040,000 acres. This is not near 51.9 million acres.]
http://www.grist.org/article/rice
61,000/3,794,083*100=1.6 percent. [If 61,000 is correct]