Design: Billboard Baudelaire

he American photographer Walker Evans was for most of his life an enthusiastic

documentarist of the vernacular: billboard signs, advertising boardings, shop fronts, road signs and cinema marquee banners are as fundamental to his art as the portraits of the Depression-era South for which he is best known. He delighted in the language of the streets, from folk art-y, handwritten inscriptions to the vast messages of mass consumerism that rolled out across the map of America in their millions.

His best photographs reveal the element that chance plays, and those moments of lyricism when commercial signs reveal their peculiar hidden messages. It comes as no surprise to learn that Evans was almost literally born into the world of graphic representation. His father, also Walker Evans, began a career in advertising as an agent for the Wabash Railroad Company and rose to the top of a then young profession in the automobile industry. He was also, for a time, a copywriter for Lord and Thomas of Chicago. The upbringing of Walker Evans Jnr, it has been surmised, must have contributed to the precision of his eye for a landscape of contemporary America, populated not just with the poor and dispossessed but with an alphabet that often spoke up for them in more subliminal, enduring and effective ways.

Not content with photographing the pictorial insignia of the new American way in situ, Evans was also a lifelong scavenger and hoarder of rusting objets trouves. So taken was he with a Coca-Cola sign found on an abandoned roadside shack in Virginia, its marks of gradual corrosion lovingly recorded by him inch by inch, and with such care and wonderment, that several of his students banded together, drove to Virginia and brought it back for him.

He put it high up on his living-room wall. "You've got to collect," he once said in interview, "You know how a collector is. He gets excessively conscious of a certain object and falls in love with it and pursues it ... And it's compulsive and you can hardly stop."

For his show at Yale University in 1971, Evans arranged, beyond a concealing wall of the gallery, the 10 disintegrating road signs that appeared in around a third of the photographs on display. The effect was, by most accounts, stunning and one installation - a decrescendo of fading "No Trespassing" signs - resembled a series of Warhol screenprints.

"Evans has sprung a beguiling intellectual trap," wrote Hilton Kramer in the New York Times, "that aestheticians will be extricating themselves for years to come." Though Kramer was accused of hyperbole, Evans, it is generally agreed, kickstarted the debate that still rages in photographic theory: what is the relationship, if any, between the photograph and the objects that appear in it? The notion of "constructed realities" may have as its point of inception this groundbreaking show.

But it is not only for this that Evans is considered the most enduring and grandest of American documentary photographers, though threads from this part of his oeuvre can be discerned in the work of the generations that succeeded him, most notably that of Lee Friedlander, Robert Frank and the neo-colourist William Eggleston. Evans's series on the poor of Alabama began as a commission for Fortune magazine, but when it was declined by its editors, it turned into the magnificent and moving book Let Us Now Praise Famous Men. The book is perhaps the most influential of all his considerable output.

Evans's first love was writing, in particular the works of Baudelaire and the Symbolist poets. He came relatively late to photography, at 25. In a career that lasted more than half a century, he was the first living photographer to be given a show at the Museum of Modern Art in New York (in 1938), was a Guggenheim Fellow by 1940 and ended his days as a Professor of Graphic Design at Yale. It appears he never lost his youthful desire to write though he had reconciled himself with his means of expression: "I now know," he sai.d, "that Flaubert's aesthetic is absolutely mine."

But for a photographer of such international standing - for a while he was the chronicler non pareil of the visual culture of his times - his subject matter was consistently narrow and surprisingly constant: architecture, both rural and urban, industrial and domestic, preoccupied him (he had a personal fondness for pre-Civil War buildings) and portraiture: his photographs of James Agee (his collaborator on Famous Men) and of his friend, the poet Hart Crane (whose epic The Bridge he illustrated in 1930), are rightly famous. He despised the pictorialism of Alfred Stieglitz and photography that pretended to be art. When a student had written on Stieglitz, Evans had just one question for him. Did he know how Stieglitz got his money? Stieglitz had a private income all of his life and Evans wanted to be sure the student knew it. He affected to despise photography that reeked of commercialism (presumably he exempted his own 20-year career with Fortune). But perhaps this work, and his commissions for other publications such as Vogue, were, as they were to so many photographers, merely the means to an end. They financed, in Evans's case, his documents of American life. As his student and later friend and critic Jerry Thompson has put it, "The Coca-Cola corporate headquarters and its balance sheet might interest the readers and some editors of Fortune magazine, but not Evans. His interest was in the discarded pull-top from a Coca-Cola can, or the flattened can itself, and the rusted signs the company had abandoned long ago. He was not interested in the achievement of record but the incidental detritus."

Above even the billboard, Evans relished the more spontaneous and inventive evidence of traditional American life to be found in hand-painted signs. He collected by camera the shopfronts of butchers, horse dealers and barbershops as well as the means that shoe-shine boys, flower sellers chose to advertise their wares. For Evans, the photography of signs, which, when he started out in the late 1920s, predated by decades Pop Art's infatuation with signals of consumerism, was his own form of poetry. The Baudelaire, after all, of billboards

`Signs', by Walker Evans, is published by Thames & Hudson on 12 October, pounds 16.95