In 2013, it's 150 years since steam trains started ploughing underneath the streets of London on what was then called the Metropolitan Railway, the world's first urban underground line. Quietly, something equally vital to the enduringly iconic status of London's tube is marking its anniversary: 100 years ago, Frank Pick, commercial manager of The Underground Group, commissioned typographer Edward Johnston to design a new typeface for all signage on the railways.

It's hard to express quite how visually confusing the Edwardian metropolis was: advertising was plastered onto every available surface, the signs and posters themselves clogged with all manner of complicated type and clutter. To walk around the city back then was to wade through floods of information, immersed in a chaotic whirl of pedestrians, horse and tram traffic – the acceleration of the metropolis, brought about by the rise of industrial capitalism in the middle of the 19th century, was reaching dizzying speeds.

The "Underground" typeface itself (now known as Johnston) was a calm, rational, tranquil insertion into this increasingly complex world. Designed to absolutely not be mistaken for advertising, its proportions were based upon Roman precedents, with perfectly circular "o"s, while it utilised a quirky diagonal dot for periods, "i"s and "j"s. Most notably, it's a sans serif, meaning that it lacks the little flicks and terminations that adorn typical Latin alphabets. And while sans serifs do date back to ancient Europe, their use as a way to simplify, declutter and rationalise graphics on such a scale was unheard of – as Pick put it, the new type's character had "the bold simplicity of the authentic lettering of the finest periods", but was also "belonging unmistakably to the 20th century".

In fact, Johnston marks the beginning of one of the most successful modernist design projects of the early 20th century. From the outset, modernism was a two-headed project; artists, writers and designers were thrilled by the vertiginous accelerations of the capitalist metropolis, but were concerned with finding a proper way to navigate them. For every futurist manifesto, revelling in the vortex, there was an attempt to use new design languages as a way to provide better conditions for all.

After the introduction of the typeface in 1916, Pick, an enlightened despot of a client with a magnificent passion for design, introduced the world-famous bullseye logo, and by the 1920s commissioned architect Charles Holden to design a series of stations, most notably Piccadilly Circus, and the UK's very first skyscraper, 55 Broadway. Lastly, in 1933 Harry Beck's simplified tube map completed this triumphant set of design classics. It's a shame however that this particular modernism is generally not taken quite as seriously as the Bauhaus in Germany, the constructivists of Soviet Union, or many of the other avant-garde groups of the time. Because it didn't sweep away all before it, perhaps because the design and buildings weren't quite shocking enough, the huge success of Pick, Johnston and Holden's London modernism isn't held in quite the same high esteem.

But in ensuing years almost all undergrounds and transit systems would adopt a version of this approach, and it would eventually permeate industrial and graphic design almost totally. Johnston's pupil Eric Gill would develop gill sans, one of the most elegant sans serif typefaces around, and eventually Switzerland's Max Miedinger would give us the all-conquering helvetica in 1957, perhaps the world's most frequently encountered text. The spirit of this quotidian modernism was also visible in the "isotype" visual language developed by Otto Neurath and Gerd Arntz in Vienna in the 1930s, and later in Romek Marber's grid of 1961, a layout standard that ushered in the classic era of Penguin book design.

In almost all these cases there's a strong leftist current – Pick was a staunch social democrat and champion of the welfare state, Neurath and Arntz were movers in inter-war "Red Vienna", while the Pelican series of popular books on weighty subjects look now to be a high point of postwar culture, when radical authors such as RD Laing, John Berger or Marshall McLuhan could write bestselling paperbacks for the everyday audience.

In a way this democratic design is a victim of its own success – we now hardly notice that clarity of signage and instruction as we move through the contemporary metropolis, which is exactly as it was meant to be. But travel around in London and you can't help but notice that the Beck tube map is groaning under its own weight, its crystalline clarity almost gone, while with his cable car Boris Johnson has re-introduced corporate sponsorship to something explicitly designed to avoid it. And in the coming world of augmented reality, of "Google glass", who knows for how much longer we'll even need systems of signage that treat each and every citizen equally?

• This article was commissioned after a suggestion from HulloHulot. If there's a subject you'd like to see covered on Comment is free, please visit our You tell us page