But the Humber crossing came about as a purely political gesture: a government wanted to make employment and gain votes by uniting the industries and road networks of Lincolnshire and Yorkshire, either side of the estuary. The new county of Humberside was created around the bridge. This was an artificial construct that did not last: politically, Humberside has now been abolished. The bridge, opened in 1981, is quite possibly unnecessary anyway, since most traffic in that part of the country continues to run east-west rather than north-south across the Humber. The bridge remains saddled with a huge debt. But it was, and remains, a most potent symbol as well a technological marvel and a thing of beauty. It is no coincidence that was conceived by the political generation that was simultaneously responsible for Concorde.

No coincidence, either, that its record-breaking characteristics should have in turn be eclipsed by other politically-generated bridges, most notably the series of connections between Denmark and Sweden. Dissing and Weitling's suspension bridge of the Great Belt, opened in 1998, is even longer than the Humber. Attention soon passed from that, however, to the hybrid connection of the Oresund Link, a ten-mile road and rail connection with a combination of bridges and tunnels. The main bridge section (designed by a consortium including Ove Arup) is at 490 metres the longest cable-stay span in the world. It is also the strongest, since it carries both a motorway and, beneath it, a dual-track high-speed railway. This is scarcely a new concept - in 1855 the Prussian-American engineer John Roebling made a double-deck road and railtrack suspension bridge across the gorge of the Niagara Falls, and there have been plenty of other dual-mode bridges since - but the scale and ambition of the Oresund Link places it in a different league. To open it, in July 2000, the Queen of Denmark set off in one train from Copenhagen Central while the King of Sweden departed simultaneously from Malmo Central: the two monarchs, with their prime ministers in tow, met on the artificial island of Pebeholm to mark the ceremonial opening. As with the Humber Bridge - but with greater justification - there is talk of the link creating its own economic zone - neither Sweden nor Denmark, just "Oresund". The effect of building it has been compared to the demolition of the Berlin Wall.

There will always be such dreams. There are inevitably plans now to link Denmark more directly with Germany. That fades into insignificance compared to the project to build a bridge across the Straits of Gibraltar, so linking the key nations of the Magreb. It is unarguable that Andalusia has always had more in common with North Africa than with Northern Europe. Looked at coldly, the distance to be spanned is not insuperably great, and the technical problems are relatively straightforward. But politics are never straightforward: there, the desire for economic growth is balanced by the perceived problem of economic migration northwards from poor Africa to affluent Europe. When sea is to be crossed, it can to some extent be controlled. Politicians do not necessarily want all the aspects of free movement that a bridge brings. But setting aside all that, there is possibly only one more symbolically important crossing to be made in the entire world than the linkage of Europe and Africa: the linkage of America and Russia, across the Bering Strait. The economic case in those remote regions is perhaps difficult to make, but plans are progressing anyway. How, when bridges are finally designed in these places, do their designers begin to acknowledge the overwhelming importance of such connections?

Quality of design has normally been high on the agenda of such great national and international projects. The aesthetic falling-off of the post-war period took place elsewhere. It can perhaps be argued that the art of bridge design merely became overlaid and obscured by the proliferation of workaday structures thrown up by a huge expansion of road networks across the world. In road-building, as in the preceding railway and canal ages, the most original or at any rate most aesthetically convincing designs were usually generated by the most dramatic topography rather than by any overt desire for symbolism - the mountain viaducts of central and southern Europe, for instance. New fast motor roads affected bridge design in other ways. Firstly, it became commonplace for a single style - or at any rate very limited range - of bridge designs to be employed by each highway authority. This created a degree of homogeneity, but also awkward situations where standard designs are distorted to fit all situations. Secondly, the designs had to deliver their aesthetic message, such as it was, at speed. The faster the car is travelling, the narrower the motorist's angle of vision tends to be - as well as the shorter the overall viewing time.