These issues received a fresh airing in the 1980s and 1990s with something of a revival of the notion of the "inhabited bridge", and all these issues came to the fore. Some ideas were really little more than extrusions of the cityscape on either side - culverting the river and denying its existence. A derivative of this later proved successful in mending the wounds through cities made by traffic arteries: the highway is effectively culverted, and the landscape replaced over the top. Architect Piers Gough's "Green Bridge" in East London, for instance, unites a park. Other bridges unite educational campuses bisected by roads. Should the road beneath be acknowledged or denied? Some designs now incorporate pause-points, even belvederes, into otherwise enclosed bridges to provide the necessary view. A direct architectural comparison is with the endless extension of terminals at airports, usually involving the construction of covered bridges. Again, the earlier tendency to keep such links fully enclosed and artificially lit has been replaced by a new aesthetic of transparency. Those who regard inhabited bridges as rare should look at airports with fresh eyes.

At the start of the 21st century, then, bridges have now become accepted symbols of regeneration and revived civic pride, working on the micro scale to connect parts of the urban fabric. The contrast is with those bridges that work on the macro scale to connect nations, even continents. It might seem a paradox that, as some bridges have got steadily bigger, others have simultaneously become a great deal smaller, but both are aspects of the same phenomenon. There is, perhaps, a greater emphasis on urbanism, on the space between buildings and districts. It has also been well noted that a bridge, because it facilitates movement, can help to correct the economic gradient between rich and poor sides of town. When a bridge is flung across the dividing line - be it a waterway, road or railway - the effect is almost hydraulic: the money starts to flow from the rich side to the poor side. This is why, in so many urban regeneration projects around the world, the bridge is an essential part of the process.

Again, this works on all scales. At one extreme, you have a modest new bridge to revitalise the part of town - any town - on the wrong side of the tracks. It might be regarded as post-industrial Band-Aid, since its task is usually to help rescue pockets of economic deprivation left high and dry by the receding tide of blue-collar jobs - but if so, then nobody can deny that the bridge, as an urban form, is peculiarly suited to this task both physically and emotionally. At the other extreme, you have something like the 1998 Jamuna Bridge in Bangla Desh: five kilometres long, with another 30 km of approach roads, crossing the braided, constantly shifting, earthquake and flood-prone course of the River Jamuna or Brahmaputra. That low, multiple-span bridge is scarcely spectacular apart from its length, but it unites the agricultural west of the country with the industrial east. Two nations become one.

The picture is of course not all rosy. Yes, a bridge can heal divided communities, but then again it can be a focus of sectarian hatred. Bridges are targets for bombers. Bridges are sought out by refugees. A bridge can offer more than it can deliver: a bridge built from nowhere to nowhere - as sometimes happens in anticipation of development - might create a sense of place, or it might remain marooned and ridiculous. A road bridge can also be too successful in sucking in polluting traffic and sprawls of buildings: like all roads, they can generate their own traffic just by virtue of existing. And a bridge, like any building, can outlive its usefulness and become an artefact of industrial archaeology. A more potent and iconic artefact, however, than many a conventional building, as Thomas Pritchard's original 18th century Iron Bridge in the long de-industrialised English county of Shropshire, or a thousand abandoned railway viaducts all over the world, bear witness.

Bridges provide a way to understand and appreciate the life of a town, city, country, or economic region. Even abandoned and derelict, they imply movement, action, prosperity, power. Bridges make, perhaps, the most evocative of all ruins. The broken, 12th-century bridge of St. Benezet in Avignon - abandoned as a crossing in 1680, so becoming a tourist magnet ever since - gives us the clue. The single most fascinating aspect of today's intense design focus on this area is therefore the beguiling notion of the bridge's function not as journey but as destination, an end in itself.