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Jock Kinneir, Margaret Calvert, Simon Morgan. Primary route sign for British roadways. Designed 1957–1967

Retroreflective sign face sheeting, composite substrate, aluminum alloy support channels, 60 × 70 × 1 1/8" (152.4 × 177.8 × 2.9 cm). Committee on Architecture and Design Funds

Margaret Calvert: My name's Margaret Calvert and I'm a graphic designer and typographer. I worked with Jock Kinneir on the UK road signs in the early ‘60s. The whole system was designed from the point of view of the driver and very important obviously was the legibility. It was absolutely stripping it down to the bare essentials.

What you're seeing is a roundabout sign. It is white on green and the route numbers are in yellow. Place names all used to be in capitals. With upper and lower case lettering, from a distance, you read the shape of a word, whereas in capitals you have to read each individual letter form almost.

The triangle was always established as a warning sign, so it's a common graphic language. And then the red and the white would reflect when the headlights hit it. And then the actual roadwork man is not reflective. And that is a very striking and strong image to alert you to roadworks. You will find that particular sign all over Europe.

The school children crossing, I actually based that on myself at that age, which would have been about eight. It was very important, I thought, to change the original children crossing one to a girl leading a small boy, because I felt it made it more caring, as opposed to two of the same age.

Designing is about answering the right question. You never set out to design anything iconic, but you just simply put yourself into it. If it works for you, then hopefully it works for a larger audience.