The biographer of Robert Moses, Robert A. Caro, refers to the bridges and underpasses of the famed New York State parkways. These bridges and underpasses are quite low, intentionally specified by Moses to allow only private cars to pass. All those who traveled by bus because they were poor or black or both were barred from the use and enjoyment of the parkland and its "public amenities" by the technical design of the bridges. Even at the time of Robert Moses, a political statement of the form "We don't want them blacks in our parks" would have been unacceptable in New York State. But a technological expression of the same prejudice appeared to be all right. Of course, to the public the intent of the design became evident only after it was executed, and then the bridges were there.
These days I am more interested in the reverse case [of Da Vinci's helicopter]: ideas that could have worked many centuries before they actually appeared. The economist Alex Tabarrok calls these “ideas behind their time”
Curious minds want to know why these ideas appeared so late — and whether there might be anything that would prevent delays in future. One explanation is that the ideas aren’t as simple as they appear.
The bicycle is not as straightforward an invention as it seems. To move from ox-hauled cart to human-powered bicycle requires smooth-rolling wheel bearings, which in turn need precisely engineered bearing balls. Modern steel ball bearings were not patented until the late 1700s, and demand from the 19th-century bicycle industry helped to improve their design.