The Terrafugia Transition has received exemptions from the NHTSA that make it legal to drive on the road. With polycarbonate windows instead of automotive safety glass and tires not normally considered street legal, it has nevertheless passed muster for both the sky and the highway in terms of weight and safety. Alex last reported on Terrafugia's innovation on June 29th, 2010 and the company has been doing lots of fine tuning since then to attain the proper licensing and exemptions needed to ensure these roadable aircraft are ready for delivery by the end of 2012.
http://www.popsci.com/technology/article/2011-07/terrafugias-roadable-aircraft-receives-regulatory-clearance-again-time-road