5,240 ft (1,600 m) 1,600 m long.
The Severn Estuary presented a barrier between the Bristol area and South Wales. The estuary has a maximum tidal range of 14.5 metres, the second highest in the world and during a rising or falling tide, strong currents of up to 8 knots (4 m/s). The path taken by the bridge is close to that of the Severn Tunnel which has carried the railway line beneath the river bed since 1886. Much of the estuary is mudflats at low tide, but at high tide can be covered by up to 14 metres of water. The cable stayed section of the crossing is over 1,044.7 metres (3,427.5 ft) long, consisting of a 34.6 metres (113.5 ft) wide deck made from steel plate girders with a composite reinforced concrete slab. These were prefabricated on shore and put in place using balanced cantilever methods. The path taken by the bridge is close to that of the Severn Tunnel which has carried the railway line beneath the river bed since 1886. Much of the estuary is mudflats at low tide, but at high tide can be covered by up to 14 metres of water. The first proposal for a bridge across the Severn, approximately in the same location as that eventually constructed, was in 1824 by Thomas Telford, who had been asked to advise on how to improve mail coach services between London and Wales. The triangulation this offers is an attempt to reduce vibration, as is the prominent use of Stockbridge dampers on the cables. The bridge is 5,240 ft (1,600 m) long, consisting of a 3,240 ft (988 m) central span between the towers and the two 1,000 ft (305 m) side spans. The towers rise to 445 ft (136 m) above mean high water and are of hollow box construction The highest, tallest, longest and oldest bridges in the world. Picture: ANTHONY MARSHALL. However, the longest British bridge in its entirety is the Second Severn Crossing, which has a length of roughly 3.2 miles, over twice as long as the Humber Bridge. The Second Severn Crossing carries the M4 motorway over the River Severn between England and Wales.  The Wye Bridge is a 1,340 ft (408 m) long cable-stayed bridge, which crosses the border marked by the River Wye between England and Wales, 2 miles (3.2 km) south of Chepstow. It consists of a single large cable stayed section with two single-leg pylons supporting the bridge deck from the centre of the roadway. The triangulation this offers is an attempt to reduce vibration, as is the prominent use of Stockbridge dampers on the cables. The bridge is 5,240 ft (1,600 m) long, consisting of a 3,240 ft (988 m) central span between the towers and the two 1,000 ft (305 m) side spans. The towers rise to 445 ft (136 m) above mean high water and are of hollow box construction In 1966 the first Severn road bridge, a four-lane suspension bridge was opened carrying the M4 motorway from England into South Wales. By 1984 traffic across the first Severn Bridge had tripled and it was projected that by the mid-1990s the old bridge would be running at capacity. The path taken by the bridge is close to that of the Severn Tunnel which has carried the railway line beneath the river bed since 1886. Much of the estuary is mudflats at low tide, but at high tide can be covered by up to 14 metres of water. The Severn Bridge (Welsh: Pont Hafren) is a motorway suspension bridge spanning the River Severn and River Wye between Aust, South Gloucestershire (just north of Bristol) in England, and Chepstow, Monmouthshire in South Wales, via Beachley, Gloucestershire, a peninsula between the two rivers. The triangulation this offers is an attempt to reduce vibration, as is the prominent use of Stockbridge dampers on the cables. The bridge is 5,240 ft (1,600 m) long, consisting of a 3,240 ft (988 m) central span between the towers and the two 1,000 ft (305 m) side spans. The towers rise to 445 ft (136 m) above mean high water and are of hollow box construction You can travel to Wales quickly and easily. It’s a big plus point. Thanks to the M4 and the Severn Bridge, we’re within three hours drive of London. Take the train, or start from Birmingham, Manchester or Liverpool, and you can be here in just a couple of hours. Flying is an option, too. 
https://en.wikipedia.org/wiki/Second_Severn_Crossing https://en.wikipedia.org/wiki/Second_Severn_Crossing https://en.wikipedia.org/wiki/Severn_Bridge http://www.telegraph.co.uk/finance/property/pictures/8997330/The-highest-tallest-longest-and-oldest-bridges-in-the-world.html?image=9 https://en.wikipedia.org/wiki/Severn_Bridge https://en.wikipedia.org/wiki/Second_Severn_Crossing https://en.wikipedia.org/wiki/Severn_Bridge http://www.visitwales.com/explore/travel-maps/uk-travel-wales
how long is the severn bridge