http://www.designfax.net/enews/20100928/feature-1.asp

Since the earliest days of the aviation industry, dozens of
patents have been filed for “road-ready” airplanes — vehicles that can fly in
the air as well as drive on the highway. However, due to the engineering
challenges involved in combining a lightweight, aerodynamic aircraft with the
stability needed for long-distance driving, none of these plans has been
successfully brought to market to date.

Thanks to Dutcher.