Are we nearly there yet? How Margaret Calvert steered Britain into the fast lane

It is almost impossible to escape Margaret Calvert. She’s standing at every motorway junction, beaming out in bold, bright letters, and at the corner of every street, warning of potential hazards ahead. Now aged 84, and still busy in her studio, the designer jointly responsible for giving British roads their visual identity is the subject of a retrospective at the Design Museum.

“I’m not quite as slim as I was back then,” says the South African-born designer, standing in front of one of her famous school crossing signs on show in the exhibition. “But the hairstyle has remained the same.” Tasked with updating the previous sign, which had depicted a grammar-school boy in a cap leading a younger girl with a satchel across the road, she decided to flip it around and put the girl in charge. She modelled the silhouette on a photo of herself as a child. Her neat bob hasn’t changed much since – nor has her ability to lead the way.

The exhibition, arranged around the atrium walls of the London museum, is timed to coincide with the launch of Calvert’s latest project, which reimagines the identity of Britain’s railways. Unveiled this week, Rail Alphabet 2 takes the typeface she originally designed for British Rail in the 1960s with the late Jock Kinneir and updates it for the digital age, making it lighter, crisper and more legible.

“There’s a muddle of different fonts used on railway signage, which are hard to read and confusing for passengers,” says Peter Hendy, chair of Network Rail. “So we were keen to work on a clean and consistent design to make journeys and stations better.” Drawings in the exhibition show how Calvert began, as always, by drawing the letterforms by hand and painting them in with gouache, before the typeface was digitised and developed in collaboration with a former pupil, Henrik Kubel. Having originally trained as an illustrator, Calvert has always placed the highest importance in the process of physically drawing the letterforms. “Despite being a lengthy process, it was my way of injecting a personal touch,” she says, “avoiding any eccentric mannerisms that might easily date.”

Across the atrium, her original gouache paintings for the motorway road signs look as fresh as ever. Not much larger than sheets of A3 paper, they are as precise as something from an ink-jet printer, leaping out of the vitrine like bold pop art. Accompanying photographs show what a mess British road signage was before Calvert and Kinneir took their sharply honed scalpel to it. “A jumbled jungle of words” was how typographer Herbert Spencer described the competing melee of road signs in 1961, “an extraordinary barrage of prose”. With the imminent arrival of the country’s early motorways, something had to be done.

Legibility at speed was the key aim. A wonderful photograph from the archive shows flat-capped men from the Road Research Laboratory sitting in an airfield as a car approaches, with a road sign for Oldham and Smethwick strapped to its roof. The idea was to ensure that the overall shape of the placename was legible from afar, rather than the individual letters needing to be identifiable. Calvert has always compared it to looking at a Rembrandt painting: “If you’re close up it doesn’t make any sense, but it all comes together at the appropriate reading distance.”

The response to this radical new sans-serif world, which had been heavily inspired by German modernist graphic design, wasn’t universally positive. The old guard were furious. Calligrapher and stonemason David Kindersley, who had designed the previous road signs’ MOT serif typeface, wrote letters to the Times complaining about these new “signs as big as houses” and arguing that his capitalised font took up much less space. But Calvert and Kinneir won out, and their work has endured.

“It embodies the ultimate goal of typography: supreme clarity,” says writer and graphic designer Adrian Shaughnessy. “Its functionality is beyond question. Companies spend all their time and energy updating their brands, but the road signs have never needed to change in all these years. It was super effective from the start.”

The exhibition takes in much more than just the road sign story. It begins with the duo’s work for the P&O ferry company, producing a series of luggage labels designed with bright colours and shapes that could be read by people of different languages and levels of literacy around the world. Their logo for Glasgow airport was equally ingenious, combining directional arrows of travel with the Scottish flag. There is also a fascinating section on their proposal for the visual identity of the French new town of St Quentin-en-Yvelines in the 1970s. They developed a series of sculptural, three-dimensional letterforms that could be read from multiple angles, and a punchy slab-serif typeface. But it was all rejected by the town as being “too English”. Now known as Calvert, the typeface was instead taken up by the Newcastle Metro, its strong, square M on a yellow background poking up as a recognisable landmark across the city. Celebrating its 40th anniversary this year, the Tyne and Wear Metro still uses the Calvert typeface, as do bus and ferry networks in the north-east.

But perhaps the most revealing part of the show is the display in one corner that serves as a testament to Calvert’s teaching career. She taught at the Royal College of Art for almost 40 years, serving as head of graphics from 1987 to 1991, and minting an entire generation of successful graphic designers in the process. Beautifully typeset tributes from staff and students, presented as a leaving gift in 2001, sum up the impact she had on the world beyond the motorways. “Margaret Calvert: Frank, fearless, innocent, vague, funny, direct, ageless, eccentric, opinionated, popular with students,” says one.

She can be pretty fierce too. “Whatever you do,” advised one colleague as I arrived at the exhibition, “don’t make the joke about the roadworks sign looking like a man wrestling with an umbrella.”

• Margaret Calvert: Woman at Work is at the Design Museum, London, until 10 January