Severn Bridge

Listen first. Under the steady hum of tyres there is the wind, and it rarely stops here. It comes up the estuary from the open sea and pulls at your jacket. Below, the water is brown and fast, thick with silt. At low tide the mudbanks shine like wet slate. The air smells of salt and river mud. This is the Severn Bridge, and to understand it you have to understand the water it crosses.

The River Severn is Britain's longest river. Here, near its mouth, it becomes something stranger. The Severn Estuary has one of the largest tidal ranges in the world. The water can rise and fall by well over ten metres in a single tide. Twice a day, enormous volumes of seawater push inland and then drain away. For centuries, this made the crossing between southern England and Wales slow, difficult and sometimes dangerous.

Before the bridge, travellers had choices, and none of them were quick. You could drive the long way round, north through Gloucester. You could take a train through the Severn Tunnel, a Victorian railway tunnel under the estuary that opened in the 1880s. Cars could even ride through it on special trains. Or you could queue for the small car ferry between Aust, on the English side, and Beachley, the narrow peninsula near Chepstow. The ferry depended on the tide and the weather. On a bad day, it did not run at all.

Here is a detail that surprises many visitors. Near the old Aust ferry slipway, in 1966, a photographer took a well-known picture of Bob Dylan. He stands in front of a poster on the ferry terminal, during his tour of Britain that year. Behind him, the new bridge was almost finished. The ferry service closed soon after the bridge opened. That photograph caught the last days of one way of crossing, just as another was arriving.

Queen Elizabeth the Second opened the bridge on 8 September 1966. At that time, the new road across it was part of the M4 motorway, the main route linking London with South Wales. The opening was a national event. People described the bridge as a gateway to Wales. For many Welsh families, it cut hours from trips to England, and for English holidaymakers it brought the coast of Pembrokeshire suddenly closer.

Now look at the deck, the long road surface hanging between the towers. It is thinner than you might expect. This is the most important thing about the bridge. Earlier suspension bridges usually hung their road from deep steel trusses, like heavy open frames. The engineers here took a different approach. They used a slim, hollow steel box, shaped in cross-section a little like an aircraft wing. The design team came from the firm Freeman Fox and Partners.

Why a wing shape? Because wind is the great enemy of suspension bridges. Engineers everywhere remembered the Tacoma Narrows Bridge in the United States, which twisted itself apart in a moderate wind in 1940. The team behind the Severn Bridge tested models in wind tunnels. They found that a streamlined box let the wind flow smoothly over and under the deck. It was lighter, cheaper in steel, and more stable. Engineers around the world copied the idea, including on the Bosphorus Bridge in Istanbul and the Humber Bridge in northern England.

The second surprise is in the thin steel rods that hang from the main cables. On most suspension bridges, these hangers drop straight down. Here, they lean at an angle, forming a zigzag pattern along the whole length of the deck. The engineers hoped this would help damp movement in the wind. It was an unusual choice. Over the decades, the angled hangers carried heavy stress, and many have been inspected and replaced. Still, the zigzag remains the bridge's clearest visual signature.

The main span between the two towers is close to one kilometre long. The towers themselves are tall, slender steel structures, not stone or concrete. Each one stands on a pier in the estuary. The great main cables pass over their tops and anchor into heavy blocks on the shore. In fact, the full crossing is more than one bridge. It includes viaducts on both sides and a separate, smaller bridge over the River Wye, close to Chepstow.

And here is something many drivers never notice. The main span over the Severn does not actually cross from England into Wales. Both Aust and the Beachley peninsula are in England. The border with Wales follows the River Wye instead. So the moment you truly enter Wales comes on the smaller Wye Bridge, just before Chepstow. The famous view of the big span is an English view. The Welsh welcome comes a little later.

Life has not been easy for the bridge. Traffic grew far beyond what its designers expected, and lorries grew heavier. In later decades, engineers found corrosion inside the main cables. To protect them, they installed a system that blows dry air through the cables, keeping moisture out. In 1996, a second crossing opened downstream, now called the Prince of Wales Bridge. The motorway moved to the new bridge, and this older crossing became the M48. Its tolls, collected for decades, were removed in 2018.

The bridge is now a protected building, listed at the highest grade for its historic and engineering importance. That status puts it alongside cathedrals and great country houses. It may seem strange for a motorway bridge. But walk out onto it, and the reasons become clear. There is a footpath and cycle path along the deck. Up here, you feel the slight flex of the structure as heavy vehicles pass. You hear the cables sing faintly in the wind. You see how little material holds so much weight in the air.

From the walkway, look out across the estuary. On a clear day, the newer bridge stands downstream, with its cable-stayed towers fanning out. Upstream, the water narrows towards the hills of the Forest of Dean. Birds work the mudflats at low tide. Below you, the Wye joins the Severn near Beachley, two rivers meeting under one road. The tide may be racing in or pouring out as you watch.

Before you leave, stand still for a moment. Feel the wind pressing on you, the same wind the engineers tested in their tunnels. Look along that zigzag of hangers, fading into the haze over the water. Every car that passes is making a crossing that once needed a ferry, a tide table and patience. As you walk back towards Chepstow, notice how the bridge looks lighter the further you go... a thin grey line across a wide, moving river.