In 1931, Harry Beck drew a diagram of the London Underground (aka the Tube), making visual sense of an extremely complex system. Today Beck's map is displayed in the Museum of Modern Art, and is considered a classic work of design. Beck's work is the template for modern subway and train maps, and his influence is seen worldwide in the way we conceptualize public transportation.
Beck's central breakthrough was making a map that simplified geography to achieve maximum clarity. He reasoned that the specific geography of each place didn't matter as much as which stop was next on a given line. So he ruthlessly simplified how each route was drawn, showing the routes as straight lines that ran only vertically, horizontally, or at 45-degree angles (even though the real lines were far more complex, wandering around the city along curving routes). He also distorted geography, so the most-traveled stops were large and clearly visible; previous maps had crammed them all together in a small space -- because that's where they were geographically. Beck's map allowed riders to solve the central problem of public transit: if I'm "here," which line do I take to get "there," and how many stops will it take? Below is an illustration of the actual geography of the tube's Zone 1 (left) versus Beck's simplified map (right):
A BBC Four documentary about Beck's map was recently posted on Smashing Telly (an excellent video blog in its own right). The film is about twenty-five minutes long, and tells the history of this map, and how it has influenced design. Go watch the documentary if you're interested in design, history, or public transportation. (Note: Smashing Telly has broken the documentary into little segments, linked together in a sort of video mosaic. Start at the upper left, and it'll play through.