MORE THAN JUST A BRIDG; E OVER TROUBLED WATERS

Inhabited bridges are rare things. There are just two in Britain, of any scale. Yet there is something irresistibly fascinating about them; they are, in the best sense of the word, curiosities. What would it have been like to live or work on London Bridge when it was packed with shops and houses and was decorated with the bloody heads of traitors impaled on spikes? How would it have felt to cross the inhabited bridges of Bath (Pulteney), Florence (Ponte Vecchio) or Venice (Rialto) before their role was diminished, leaving them with little more than souvenir shops?

Such musings are encouraged by a delightful new show, "Living Bridges", which opens this week at the Royal Academy in London. Visitors to the exhibition will walk beside a "River of Time", awash with real water, and discover the history of the habitable bridge, from old London Bridge (the one with a tendency to fall down) to a sequence of intriguing new designs by cutting-edge contemporary architects. Each bridge is beautifully realised as a large-scale (1:200) wooden model. The exhibition has been designed by Nigel Coates of Branson Coates Architecture, who has his own protuberant bridge design along the River of Time in the guise of a giant cock and balls thrusting its saucy way across the demure Thames.

The River of Time flows to a halt with seven would-be bridges by architects drawn from across Europe. These are the entries to a competition sponsored by Thames Water for the design of a brand new inhabited bridge across the Thames, which may yet span the river from Temple Gardens on its north bank to the area in front of the horrid London Weekend Television building to the south. The winner will be announced this week. Quite why London needs an inhabited bridge to block the glorious view of St Paul's cathedral from Waterloo Bridge is anyone's guess, but the ideas are fascinating, ranging from an ungainly design by Antoine Grumbach (France) to a graceful arch designed by Future Systems (UK).

The juxtaposition of old and new is particularly fascinating. What, one wonders, would we make of such bravura designs as Sir John Soane's extraordinarily ambitious inhabited bridge if they were submitted today? Architects have been fascinated with the idea of bridges as buildings for centuries, and the exhibition includes designs by, among others, Sir Edwin Lutyens and Frank Lloyd Wright. Will Alsop's proposal for a bridge to suspend a relocated ICA over the Thames is one of the most daring of the latest wave of bridge designs and one that would doubtless be very popular, not only linking the two sides of a river but also spanning the cultural void between contemporary arts and a large public.

The exhibition, which is sponsored by the Independent, is the idea of Jean Dethier, of the Centre Pompidou in Paris, who also contributes to the informative catalogue. He has been researching these fabulous (and occasionally falling) structures for many years, and probably knows more about the habitable bridge than anyone else alive today. Perhaps one day he will live on one.

`Living Bridges': Royal Academy of Arts, London W1 (0171 494 5615), Thursday to 18 December; 10am-6pm daily (last admission 5.30pm).