I think that I'd like to introduce a subset in my Bad Ideas category: Bad Flat Ideas. To start it off, I found this very lovely example in LIFE magazine for 18 March 1946, the brainchild of generally-appropriate real estate mogul William Zeckendorf: a spectacularly bad idea of installing a rooftop airport along a very long swath of property along the West Side.
It seems as though the airport would extend from the bottom third of Central Park south to, well, I really can't tell, perhaps lower midtown? Honestly, it doesn't matter. For all of its size it actually isn't quite big enough, looking like there's maybe 10 or 20 plane lengths margin-of-error at the end of the runway. Plus there's the *no* margin of error bit for the edges of the airport, which assumes that no plane will ever topple over the table top. Plus a host of other pertinent stuff.
The penultimate line: it is also incredibly ugly, a definition of architectural ugliness that few projects could match.
The bottom line though to this project was the bottom line: in the end, the Manhattan Airport would save about 15 minutes travel time to Philly over Laguardia.
I should not leave unmentioned the unmentionable project proposed for poor London (which I wrote about late last year here):