("stuart" wrote)
But what would prevent a design (even low performance/ultra light to
start) that would allow a pilot to fly into an airport, then fold up his
wings and cruise down surface streets at 45mph?

I would start with the car - assuming not a motorcycle. What is the lightest
car you can build, that is street legal, will fit your mission and can
cruise at 50 mph? From there design your plane around THAT.

http://www.goldenwingsmuseum.com/Air...ero%20Car.html
http://www.goldenwingsmuseum.com/Photo's/Aero%20Car-2.jpg
At Golden Wings Museum: (ANE) Anoka-Blaine Airport, MN

Good luck.

Montblack