Benjamin Franklin Bridge

Stand still for a moment near the Camden end of the bridge, and listen. Above you there is a steady hum: tyres on the deck, the low rumble of traffic crossing the Delaware River. Every few minutes a deeper sound joins it. That is a train, running right across the bridge between New Jersey and Pennsylvania. Look up and you will see the steel painted a soft, pale blue, rising in long curves toward the towers.

This is the Benjamin Franklin Bridge. It connects Camden, on the eastern bank of the Delaware, with Philadelphia on the western side. For most of its life, people on both shores have treated it as a simple fact of daily life. It carries commuters, students, delivery vans and the occasional runner. But when it opened, it was one of the most ambitious pieces of engineering anywhere in the world.

The bridge opened to traffic in 1926. The date was chosen with care. That year was the 150th anniversary of American independence, and Philadelphia, where the Declaration of Independence was signed, held a large celebration. A new crossing over the Delaware fitted that mood of confidence. For its first three decades it was called simply the Delaware River Bridge. It took the name of Benjamin Franklin in 1956, the 250th anniversary of his birth.

Here is the first surprise. When it opened, this bridge had the longest suspension span in the world. Its main span, between the two great towers, is about 533 metres long. That record did not last long, because other bridges in North America soon passed it. But for a few years, the longest gap ever crossed by a suspension bridge was right here, between Camden and Philadelphia.

A suspension bridge works in a simple way. Two huge main cables hang over the tall towers, in a long, graceful curve. Smaller vertical cables drop from them and hold up the road deck. At each end, the main cables run down into heavy anchorages, which are massive blocks of masonry and concrete. These anchorages hold the whole structure in tension. If you look at the Camden end, you can see how much stone it takes to keep a bridge like this in place.

The chief engineer was Ralph Modjeski, one of the leading bridge builders of his era. He worked with the architect Paul Philippe Cret, a French-born designer who taught in Philadelphia. Cret shaped the look of the bridge — the clean towers, the stone anchorages, the restrained ornament. His idea was that an engineering work should also look calm and dignified. The result is a bridge that feels solid rather than showy, even though its scale is enormous.

Before the bridge, crossing the river usually meant taking a ferry. Ferries had run between Camden and Philadelphia for generations, and on busy days the queues could be long. The new bridge changed that almost overnight. Cars could cross in minutes. Over the following years, towns in South Jersey grew as people found they could live on one side and work on the other. In a quiet way, this bridge helped to shape the suburbs around you.

Now listen again for the trains. Those tracks on the bridge carry the PATCO Speedline, a rapid transit line linking South Jersey with Center City Philadelphia. Rail service across the bridge is older than PATCO itself, and the tracks have been part of the bridge's story for most of its life. Many visitors drive across without realising that trains share the deck. If you walk the bridge, you will feel them pass beside you, a few metres away.

And you can walk it. A pedestrian walkway runs along the bridge, and it is one of the best ways to understand the structure. On the Camden side, the entrance is near the downtown area, a short walk from the city's streets. As you climb, the river opens up beneath you. The walk is long and rises steadily toward the middle, so take your time. Check access before you go, because the walkway sometimes closes for maintenance or events.

Halfway across, stop and turn around. Behind you lies Camden, with its waterfront, its university buildings and the low roofs of the older neighbourhoods. Camden has a long industrial history. Shipyards, factories and the recording industry all once played a large part here. The poet Walt Whitman spent his final years in Camden, and his house still stands in the city. From the bridge, you can see how closely this city is tied to the river.

Ahead of you is Philadelphia. The skyline of Center City rises behind the old riverfront, and the colonial streets of the historic district sit close to the western end of the bridge. On a clear day you can see the river bending to the south, with ships, tugs and small boats moving along it. The view shows something easy to forget at street level: these two cities face each other across a working river.

Here is the second surprise. At night, the bridge becomes a light display. Lights were added along the cables in the late 1980s, for the 200th anniversary of the United States Constitution, which was written in Philadelphia. Today the lighting can change colour and pattern, sometimes for holidays or local sports teams. From the Camden waterfront after dark, the long cables seem to be drawn across the sky in light.

The bridge is maintained by the Delaware River Port Authority, which looks after several crossings in the region. Keeping a structure like this safe is constant work. Painters, cable inspectors and engineers move along it throughout the year. The pale blue paint you see is not only for appearance. It protects the steel from rain, river air and the slow work of rust. A bridge of this age stays useful only because people care for it every single day.

Before you leave, look once more at the towers. Notice how the main cables dip low at the centre and rise again at each end, like a line drawn by a steady hand. Listen for the next train, and for the traffic that never quite stops. Generations of people have crossed here to work, to study and to visit family. As you walk back into Camden, you are moving along the same path they took, between two cities joined by one long span of steel.