Its first section opened in 1973, and the rest was all built, but the A41(M) doesn't exist. It was canned in favour of orbital roadbuilding, and later resurrected in a more modest form.
Today two motorways cross the Severn near Chepstow. But as recently as 1965, the only crossing was a ferry that carried six cars at a time, from Aust to Beachley.