Transit signage is an especially fascinating genre. The graphics have to relay loads of information to a great number of people. When done right, you don't "notice" the design, you just get from point A to point B hassle-free. So I really enjoyed the capsule history of London's Underground logo summarized in CoDesign: "The Roundel is neither picture nor typography. The Roundel is always instantly recognizable as itself in whatever context it is placed while simultaneously communicating the idea of transportation, urban cool, and serving as a visual metonym for London itself," writes Logo for London author David Lawrence. For more on transportation signage, view my story on New York's WalkNYC design by Pentagram.