If you think about it, traffic signs should be invisible. Not see-through invisible, but intuitively invisible; if they work like they’re supposed to, you won’t even realize you’re using them.

That was the challenge put to Lock Kinneir and Margaret Calvert in 1958, when the British government hired the London designers to devise a new signage system for the country’s roads. Up until then, traffic wayfinding in the UK had been aided through a slapdash series of roadside posts, cobbled together in various colors, fonts, and type sizes. The system was so bad that the design magazine Typographica published two photo essays condemning its terrible usability.

Transport Medium lowercase for use on dark background signs Jock Kinneir and Margaret Calvert, Courtesy of London Design Museum

If longevity is any measure, the Kinneir-Calvert signage system worked: It’s still in place today, and has informed a number of other similar projects around the world. “They work really on a subconscious level,” says Patrick Murphy, a London designer who, for the anniversary of the road signs, organized an exhibit about them at the London Design Museum. An accompanying book called Redirections will be out later this year. This was the first time anyone had recorded the project’s backstory for posterity. Which is surprising, maybe, given just how many lives it has directly affected. “People experience these things more than luxury design, but it’s not like Apple with Jonathan Ive. People don’t know Calvert and Kinneir as designers,” Murphy says.

As obvious a civic need as consistent traffic signage system seems today, it wasn’t up until the 1950s. That’s when the auto industry started manufacturing cheaper cars, luring railway passengers away from mass transit, causing road traffic to surge. The government launched a massive construction project to start building high-speed motorways. Drivers would need to read signs faster than before, and the old, ad-hoc system wouldn’t cut it.

Transport Medium for use on dark background signs Jock Kinneir and Margaret Calvert, Courtesy of London Design Museum

Kinneir and Calvert were the natural choice for the job. A few years earlier, Kinneir had won his first signage gig, for the then-new Gatwick Airport through sheer serendipity; he met the airport’s architect in a chance bus-line encounter. Wayfinding systems weren’t a thing then; in many regards, Kinneir had to invent the signage from the ground up. Other jobs followed, ultimately leading to a major commission from the British government’s Ministry of Transport.

The constraints for the project were set with two simple questions: what do you need to know while traveling at speed, and at what distance do you need to know it? In an essay published by the Design Museum, Calvert said: “‘Style never came into it.’” The signs had to display reduced forms that could impart information immediately. Certain advances, like the new reflective white material that would coat signs, would help, but Kinneir and Calvert had to design their own typeface. In an essay Calvert wrote for the occasion, she explains:

Important details, such as the curve on the end of the lowercase l…and the obliquely cut curved strokes of the letters a, c, e, f, g, j, s, t and y, were specifically designed to help retain the word shape of place names when slightly letter-spaced…This specific letterform, after two attempts, and in two weights, was officially named ‘Transport’.

Kinneir and Calvert created rules for traffic signs that have endured to this day. Consider the wide gaps in letter spacing typically seen on roadside signs: That spacing is derived from research the designers conducted on how type should scale according to the speed of traffic and the amount of information on display. For Transport, the unit of measure for spacing is based on the width of the capital letter ‘I’—a consistency in form which, over time, helped foster a sense of familiarity in drivers.

Even the methods used to test the signs were experimental. In 1959, drama ensued when a local designer named David Kindersley petitioned that his own typeface be featured on Britain’s roadways, instead of Kinneir and Calvert’s. Kindersley’s was an uppercase typeface with serifs, created specifically for black-on-white letters—the same high-contrast combination used for the British road signs. He managed to attract enough attention to garner a face-off between his typeface and Transport:

Tests were soon initiated by the Road Research Laboratory to settle the issue. Rather comically, several volunteer airmen from Benton airport in Oxfordshire found themselves seated on a tiered platform, in the middle of the airfield, while a car drove towards them with alternate combinations of signs mounted on the roof; composed of place names in Kindersley, Transport, and for good measure the 1933 Johnston-based standard, still to be found in parts of central London. Ironically, Kindersley’s serif-ed letters proved to be 3 percent more legible than Transport—a negligible amount given the unrealistic conditions governing the tests. As a consequence, the ultimate choice rested on appearance. In the words of one observer, ‘Kindersley’s letters were just so ugly.’

After all the experimental methods, and rudimentary proof-of-concept tests, the British Road Signs have endured. You can see the signs in the London Design Museum, or, of course, on Britain’s highways.

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The Little-Known Story Behind Britain’s Road Signs