A special set of tires are fitted to the Transition, which is rated for highway speeds, but it's also designed to take the abuse that comes with multiple takeoffs and landings. Additionally, the cockpit is not wrapped with traditional laminated automotive glass. That would add weight, and also severely affect the pilot's vision in the event of a bird strike. Polycarbonate windows are the preferred porthole material.
The DOT and NHTSA exemptions are a positive step forward for Terrafugia. Unfortunately, the company has experienced a half-step backwards. A few design issues coupled with third-party supply restraints have pushed Transition production backwards. Initially, Terrafugia hoped to display a production Transition in 2011. Now, however, it seems that 2012 will be the year for our Jetson's dreams to inch closer to reality. In the interim, we'll have to be happy renaming our Roomba, Rosie.