Bridges for the city of dreams city

What if? What if there were a bridge that would let us cross this valley or that river at exactly the point we would like it to? We can all imagine new bridges, and many of us will have our own ideas as to where they should spring from and where they should touch land again. We built them across streams with sticks as children; we sang "London Bridge is falling down" - although whenever I crossed it on a red double- decker, it looked sooty but perfectly sound.

Bridges are, of course, far more than a matter-of-fact way of crossing physical voids; they are the stuff of dreams and the imagination. They are highly symbolic. We talk of building bridges with other people, countries and organisations. New Yorkers speak of suburbanites who invade Manhattan on weekends as "bridge and tunnel" folk, because it is by these means that they invade the citadel of skyscrapers and chic shopping. And, no matter how cloying the saccharine falsetto of Art Garfunkel singing "Like a Bridge Over Troubled Water, I Will Lay Me Down", we cannot help but understand the sentiment.

We are fond of bridges, and appear to accept them for what they are, whether they are garbed in Neo-Gothic fantasy dress, like London's Tower Bridge, or express the latest developments in structural engineering. Only the dullest human being remains unmoved or unexcited by bridges. And each crossing remains an adventure. Which north Londoner can cross Tower Bridge, with that thrilling, shifting inch-gap in the middle through which the grey-brown tidal waters of the Thames can be seen churning, without a feeling of trepidation: at the end of the bridge is "sarf" London. Do we need a passport? Here be monsters.

Given our fascination and even love of bridges, the Royal Academy of Arts can only succeed with Living Bridges, an exciting exhibition of that most intriguing of all river crossings, the inhabited bridge - like old London Bridge, Rialto Bridge, Venice, Ponte Vecchio, Florence, Pulteney Bridge, Bath - a feat of engineering that we can live, work and shop on as well as cross.

Living Bridges began life in Paris as an over-the-shoulder look at the inhabited bridge in history. Jean Dethier of the Pompidou Centre planned a exhibition of paintings depicting inhabited bridges through history. Because the Pompidou Centre will be closed for major repairs and renovation until the turn of the century, he brought the idea to London, where the Royal Academy has transformed it into a show that combines contemporary wizardry and a sense of occasion, drawing on years of detailed research by Jean Dethier. Peter Murray and Mary Anne Stevens, curators of the exhibition, appear to have successfully bridged the gap between the popular touch and scholarship.

Nigel Coates, of Branson Coates Architecture, has designed the show, and it it promises to be quite breathtaking. Coates has arranged 21 spectacular, purpose-built wooden models of inhabited bridges (made by Andrew Ingham & Associates) across a miniature river that snakes its way through the RA's principal galleries. The water coursing under the magnificent bridges has been dyed grey-brown to match the natural colour of the Thames.

The models begin with Old London Bridge (the one that was always falling down) as it was in 1600, when it was decorated by the decapitated heads of traitors (a German visitor counted more than 30 in 1598) and continue through examples of the most famous (Rialto, Ponte Vecchio, Poulteney) and others that remained on architects' drawing-boards. These include Sir John Soane's triumphal river crossing (an RA Gold Medal student award entry) from 1776, Sir Edwin Lutyens' design for an art gallery bridge across the river Liffey in Dublin, and a wilfully ambitious scheme for a crystal Tower Bridge from the Thirties.

The exhibition is brought up to date with the seven invited entries to the Thames Water habitable bridge competition, one of which, the judges hope, may be built across the Thames from Temple Gardens to the London Weekend Television building. The entries are by Daniel Libeskind, Zaha Hadid, Branson Coates, Future Systems, Antoine Grumbach, Ian Ritchie and Krier Kohl. Although the idea of such a bridge has been enthusiastically endorsed by John Gummer and others, it raises a simple and fundamental problem. Inhabited bridges tend, by their nature, to be bulky. The view from Waterloo Bridge looking east to St Paul's cathedral and the City of London is one of the finest any capital city has to offer (finer than Wordsworth's famous view from old Westminster Bridge). To block it with a new bridge that is a delightful idea in principle, but which would be all but gratuitous in practice, seems a reactionary idea that could only provoke unnecessary anger.

This raises a further issue that Living Bridges does not raise, though there is no reason why it should. Which is this; why put so much creative energy into designing and encouraging a bridge we have no need of, when those we really do need are in dire need of a shot of imagination?

Over the past few years, the London Docklands Development Commission has been investing in a sequence of filigree pedestrian bridges, creating new and useful walkways through east London besides the Thames. Each has been a gem, designed by teams of imaginative architects and sympathetic engineers. The latest is by Future Systems, right next to The Independent's offices at Canary Wharf, and is a delight. It will not be built on.

The idea of habitable bridges appeals to our imagination, but we have to ask, why did they all but vanish if they were so delightful, and why have none of any significance been commissioned in the past 200 years? The answer is simple: inhabited bridges were inconvenient and unhygienic. Old London Bridge (replaced in 1823-31 and again in 1967-72) had already had its houses, shops and inns removed by the architects Sir Robert Taylor and George Dance (Sir John Soane's teacher) between 1758 and 1762. At much the same time, Europe's other 100 or so inhabited bridges were stripped of buildings or demolished.

In principle, the inhabited bridge is a logical use of strong, supportive structures. A bridge is a foundation of sorts and can carry vast weights. Whether we ought or need to impose buildings on them is another question. If the building is the same thing as the bridge (and vice versa), as in the examples of Lutyens' art gallery bridge across the Liffey and Will Alsop's update of much the same idea for the Institute of Contemporary Arts (ICA) across the Thames, we are on firm foundations. If the bridge is simply an excuse to build a vague gathering of unspecified shops and cafes across a major river as it passes through a city centre, then we are on shaky ground (or troubled waters).

Whatever the pros and cons of a new generation of inhabited bridges, Living Bridges should prove to be an enjoyable and thought-provoking experience. If only space could have been found to build models of Konstantin Melnikov's project (1925) to garage 1,000 cars on a new bridge across the Seine (Futurism in excelsis), or Raymond Hood and Hugh Ferris's scheme from the Thirties to build 100 new bridges each housing 50,000 people and linking Manhattan to surrounding boroughs. But, there is enough at the RA to satisfy most of us. This is a show that the child in all of us, as well as those concerned with the future of our cities, will want to return to and that, unlike Old London Bridge, has not been set up only to fall down.

Living Bridges, Royal Academy of Arts, Piccadilly, London W1, 26 September to 18 December. Open 10am-6pm daily; admission pounds 5; concessions pounds 3.50; children 12-18 years, pounds 2.50; 8-11 years, pounds 1. Catalogue, pounds xx. The exhibition is supported by the Corporation of London and the Generale des Eaux Group in association with `The Independent' and in co-operation with the Thames Water Habitable Bridge competition.

I have 25 pairs of tickets to give away to the first 25 SAEs opened next week. Write to Jonathan Glancey, `The Independent', One Canada Square, Canary Wharf, London E14 5DL.