Some diagrams fail because they do not offer any advantages over existing methods: a very small "family tree" which only covers two generations – a person and their parents – provides no benefit compared to describing the family verbally. Others fail because they over-reach themselves.
New York City's Metropolitan Transportation Authority introduced a system diagram designed by Massimo Vignelli in 1972. It was based on Harry Beck's 1931 design for the London Underground. But New York City's rail engineers had built a system that could not be well diagrammed. The tangle of rail lines built through Lower Manhattan remained a tangle despite being schematized; the artwork was cluttered; worst of all, the design failed to connect to pre-existing mental representations of the city's bizarre geography. The MTA retired the Vignelli diagram in 1979 in the face of public criticism.
New York City's current subway diagram, which is like a skin superimposed on a simplified map of the city, is an easier connect between its designer and its readers. Even tourists find it easier to find their way with the current map.
