Visual reference to the past is also made with the current generation of floating, pontoon bridges - Professor Eiichi Watanabe of Kyoto University has proposed a coastal road bridge consisting of a snaking box-girder deck placed on cylindrical floating concrete pontoons, for instance. The Romans would have appreciated the idea. Similarly with the latest ribbon-bridges. Instead of a footway of lively planks as in the past, you have a single narrow band of thin reinforced concrete held in tension. As with so much to do with bridge design, this type of crossing shares technology with the design of roofs - such as Alvaro Siza's virtuoso catenary-curved thin-shell concrete roof at Expo '98 in Lisbon, Portugal. Structurally, large clear-span roofs and long bridges can be near identical: the difference is that the roofs of buildings seldom have to be designed to bear constantly varying loads of traffic as well as wind. Aesthetically, bridges can learn from the buildings comparison. Too little attention is often paid to the visual effect of the undersides of bridges. In buildings, naturally enough, the roof soffit receives a high degree of design attention.

As Matthew Wells recounts in this book, ways of building bridges were refined immensely during the 20th century - taking great leaps forward during the war years, when technology transfer from other industries such as shipbuilding was at its most intense. New ways with old materials, the arrival of new materials, the increasing importance of aerodynamics, and above all the rapid rise in computer programmes capable of "testing" designed but unbuilt structures across a range of conditions - all these things led to a point where, from an engineering point of view, bridges could be made to perform structural gymnastics. What then began to occur - a process that is accelerating today - was a reassessment of what bridges meant aesthetically, from both engineering and architectural professions.

In a sense, what has been taking place is a reassessment of the nature of bridgeness. To come back to the start of this introduction: what does a particular bridge set out to achieve? Some examples deny their bridgeness, such as the Adam Brothers' Pulteney Bridge in the Georgian city of Bath, lined as it is with small shops. Being an extruded street, like the medieval Old London Bridge before it, it is perfectly possible to cross without being aware that the River Avon runs below it at all. Although you might wonder idly why the imposing, tall buildings on either side have given way to delicate, small-scale buildings - lightweight, for the time - your eye is held at street level by the standard-gauge Georgian shop windows, so the pretence is maintained. Such denial of orientation - a charge you could also level at windowless tubular-structure bridges, effectively tunnels in the air - does not apply to the Rialto in Venice. The Rialto offers you both a giveaway steep arch and open-deck pedestrian bypasses to each side of its always crowded central street. Nor does the denial of view occur in other English Palladian bridges of the 18th and early 19th centuries, where, although the bridge is treated as a piece of architecture, it is as a room with a view - and serves, in the Palladian manner, to frame and enhance that view, both from within and from without.