If you thought the old plans to pave the Hudson River and put a dome over Manhattan were, well, stupid... you ain't seen nothin' yet. Tucked in an issue of Popular Science from 1927 is a proposal that, if it had been successful, would have brought highways to our skies. Or rather, on top of Manhattan's rooftops.
"Our artist pictures here an ingenious new plan for solving NYC's traffic problems by a remarkable system of roof-top boulevards running more than sixteen miles in a straight line through the heart of the city. Bridging of cross streets for free movement of traffic; moving platforms for speedy and convenient service; healthful elevated playgrounds for children; underground railway freight service—these are some of its outstanding features."
The buildings (which would have been built from Lower Manhattan all the way to Yonkers) were to contain apartments, offices, shops, restaurants, schools, theaters, and elevators that would have carried cars between the street level and the elevated boulevard.
This came from the mind of New York-based engineer John K. Hencken, and while it never happened, it was "approved by a number of eminent engineers and city planners. They say it is entirely feasible from an engineering standpoint." [via Untapped Cities]