London Underground Tube Sign

According to WikiPedia, Transport For London’s Tube map and "roundel" logo are instantly recognisable by any Londoner, almost any Briton, and many people around the world.

The origins of the roundel, in earlier years known as the ‘bulls-eye’ or ‘target’, are obscure. While the first use of a roundel in a London transport context was the 19th-century symbol of the London General Omnibus Company – a wheel with a bar across the centre bearing the word GENERAL – its usage on the Underground stems from the decision in 1908 to find a more obvious way of highlighting station names on platforms.

The red circle with blue name bar was quickly adopted, with the word "UNDERGROUND" across the bar, as an early corporate identity.

Source: en.wikipedia.org/wiki/London_underground