At the bottom end of the range, a motorway overbridge, say, is required by the highway authorities. Or more likely, a clutch of dozens of such overbridges. At their most basic, such structures seem to be merely extruded pieces of road slab. At their best, they give a distinct character to a route: but these are the workhorse bridges, the equivalent of the hump-backed brick and stone canal access bridges of the 18th century. Low maintenance, good manners and a certain finesse is the most one can normally expect. In contrast, at the top of the range, a bridge is required that will serve a symbolic function far beyond its quotidian use. Such as the Erasmus Bridge in Rotterdam by Ben van Berkel and others, intended as a catalyst for regeneration in the run-down docks area of the city, a memorably sculpted vertical marker in the horizontal townscape. As with Florida's famous 1987 Sunshine Skyway (engineers Figg and Muller) with its yellow-sheathed cables making sail shapes across the water, the visual role of the Erasmus Bridge was clear from the outset.

The case is different - to take a current example - at the Charles River crossing north of Boston by the Swiss engineer Christian Menn, sited in an historic area where Paul Revere began his famous ride and the Battle of Bunker Hill took place. There, the task that the bridge is asked to perform is to some extent contradictory. On the one hand, it was acknowledged that this sensitive site deserved a considered, even reticent, design. On the other, this was going to be the only main route into Boston from the north. The phrase "dramatic gateway to the city" was bandied about. 16 bridge types were considered, eventually whittled down to a choice of two cable-stay options: one with a single tall landmark tower a la Rotterdam, the other - Christian Menn's solution - with smaller twin towers. This latter option was selected as the best compromise between drama and deference. Much of the aesthetic success of the design comes down to a level of detail relatively rare in highway bridge construction. Openings were made in the wide main span deck, to avoid heavy shadow on the water beneath. Cladding panels give the bridge a smooth underbelly. There is an alternation of splayed and centred cables across the spans.

Such large-scale roadway landmarks stand in clear line of descent from the barbican towers of medieval cities. The fortified bridge at Cahors in central France, with its triple tall defence towers, looks to our modern eyes as if it is crying out for cables to be slung from those towers to suspend the deck, as chains might suspend a drawbridge. The sculpting of today's support towers for suspension and cable-stay bridges satisfyingly fulfils the Cahors craving: vertical elements that not only exactly balance the horizontal, but which are also absolutely necessary to it. There is no better example of the moment of transition between the one type and the other than Tower Bridge in London of 1890 by Sir Horace Jones and J. Wolfe Barry. This was a modern steel structure combining suspended decks, steam-driven rising bascules and daringly engineered twin high-level walkways for pedestrians - yet Barry's engineering was clad by Jones in a mock-medieval granite carapace recalling those ancient bastions. What would it have looked like without the stone cladding? To judge by the internal structure, utilitarian in the extreme. Certainly nowhere near as good as the open latticework of Othmar Ammann's later George Washington Bridge over the Hudson River, the towers of which were originally intended to be clad in granite-faced concrete. The Slump of the inter-war years meant that the masonry skin was cancelled, to great visual effect.