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The page you just tried to reach at Roads.org.uk wasn't found. Either you followed a bad or out-of-date link, or you mistyped the page address.
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Your request to see a non-existent page generated the error code 404. Before you move on, you might like to read about the roads that share that number.
The A404
Unlike the document you wanted to see, the A404 is easily found. It runs from Marylebone in London to Maidenhead, Berkshire via an indirect route that takes it as far north as Amersham. At its westernmost end it forms a spur of the M4 called the A404(M). It originally ran only from London to Amersham, but was extended in 1935 to reach the A4 near Maidenhead, giving it its present rather silly route.
The route is slightly more than 43 miles in length, of which the westernmost 9.6 miles are a trunk road. There's a detailed account of its route and history at the SABRE Wiki.
You would never use the A404 to make a journey from Marylebone to Maidenhead, just like you'd never come to this page in order to find your way somewhere else. But wasn't this a pleasant diversion?
The B404
A short and insignificant route, matching this short and insignificant page, the B404 is in Covent Garden in London's West End, running from Shaftesbury Avenue to William IV Street just off Trafalgar Square. Much of it is one-way so it only forms a through route from north to south. It's described on the SABRE Wiki too.