He had been asked to advise on how to improve mail coach services between London and Wales. No action was taken, and over the next few decades the railways became the dominant mode of long-distance travel, with the Severn Railway Bridge at Sharpness being opened in 1879 and the main line Severn Tunnel in 1886. However, the growth of road traffic in the early 20th century led to further calls for improvements, and in the early 1920s Chepstow Urban District Council convened a meeting of neighbouring local authorities to consider a Severn crossing to ease congestion and delays on the A48 passing through the town. In 1935 Gloucestershire and Monmouthshire County Councils jointly promoted a Parliamentary Bill to obtain powers to build the bridge over the estuary, with 75% of costs to be met by the Ministry of Transport from the Road Fund. However, the bill was rejected by Parliament after opposition from the Great Western Railway Company.
After the Second World War, plans began to be made for a nationally funded network of trunk roads, including a Severn Bridge, for which the contract was awarded to Mott, Hay and Anderson, with Freeman Fox and Partners. The public inquiry into the scheme was held on 24 September 1946 at Bristol University. However, because Government funding was prioritised for the similar Forth Road Bridge (opened in 1964), construction of the Severn Bridge was not started until 1961: the UK government announced in 1962 that construction costs would be "recovered" by means of a toll of 2s 6d (£0.125) on all vehicle crossings, though walking or cycling across the bridge would be charge-free. The substructure was completed by contractors John Howard and Co in 1963. The superstructure contract was awarded to Associated Bridge Builders Ltd (a joint venture of Sir William Arrol & Co., Cleveland Bridge & Engineering Company and Dorman Long) in 1963, and completed in 1966.
The bridge has been featured in several promotions.
In January 1977, it was announced that bridge traffic would be restricted to a single lane in each direction following the discovery of several weaknesses in the ten-year-old structure. The lane closures would last for several months.
The Severn Bridge crossing was strengthened and resurfaced in the late 1980s as the weight of traffic grew. The work included the strengthening of the Severn Bridge towers and deck, an extension to the existing Wye Bridge towers and the replacement of the original single stays with two stays. The open structure of the new stays is designed to facilitate maintenance. Most of the strengthening work was inside the deck box and towers and so is not visible. Design of the strengthening was by Flint & Neill. The surfacing is a 35 mm (1 3⁄8 in) thick layer of mastic asphalt over an acrylic waterproofing membrane.
During its 40th year of operation in 2006, the bridge was inspected to check for corrosion of the suspension cables. According to the Highways Agency, the inspection concluded that the bridge needed restrictions on heavy goods vehicles. Such vehicles are now restricted to one lane on the bridge, with weight restriction signs in place. A system of rubber casing on the cables with dry air circulation, as used on the Forth Road Bridge, was installed in 2007–2009 in a move to halt the progress of the corrosion.