One of the most commonly cited examples of a well-designed signage system is the London Underground. The Underground's station signs consist of a red roundel and a horizontal blue bar containing the station name. They are instantly recognizable to riders throughout London, are unlikely to be confused with other signage, and can be replicated throughout printed materials, stations, and street-level entrances to provide consistency throughout the entire system. The typeface used on the signs, New Johnston, was designed in 1913 specifically for the Underground to provide readability at a distance.
The dictionary definition of sign at Wiktionary