In 1935 Sir Charles Bressey was appointed by Hore-Belisha, Minister of Transport, to make a comprehensive and systematic survey of the roads of Greater London. It was clear that the infrastructure required radical improvement to keep up with the expansion of traffic and Belisha said that Bressey's report "would stir the imagination of the whole country".
The report was published three years later and laid out a reconstruction scheme for London based on a detailed 30-year plan for highway development. Bressey's plan to deal with traffic involved tunnels, overhead roads, new arterial and circular highways and 'parkways' linking the city to the rest of the country. Before any of this could be implemented the plan was interrupted by war and aerial bombardment. Nevertheless, many of Bressey's ideas would influence post-war reconstruction and subsequent schemes for the capital's reorganisation.
Source (if you're in a UK school or library, you can access a movie about it).
Via Ptak Science Books, where you can see more illustrations of the "traffic improvements" outlined in the "Bressey Report". Check out this blog, by the way, there is much more to be found (about 900 posts on the history of ideas and technology, to be precise...). It is written and illustrated by John Ptak, an antiquarian science bookseller.
Related: Magic Motorways, a similar plan for US cities.