This page details the development of London's road plans until 1934.
London is an old city -- the last opportunity for major re-ordering of the centre had taken place after the Great Fire of 1666. However, with few exceptions, the new street plan followed the mediæval layout. The streets were narrow, bendy, and there was no planned layout such as a grid system. This meant that traffic congestion was rife -- Samuel Pepys recorded being stuck in a traffic jam (in a hackney coach) for an hour and a half.
The first bypass of London -- the wide New Road connecting Paddington and Islington (now Marylebone Road) -- was started in 1756, so that through traffic could avoid the narrow streets in the city centre. At the time of its construction, it formed the northern boundary of the built-up area, but this didn't last long.
Fast-forward to the beginning of the twentieth century... Up until now, conditions had stayed fairly similar, as traffic was either horse-drawn, or pedestrians. However, the motor car started to rear its ugly head. Apart from the fact that it could go so much faster (but obviously had the potential to be held up by horses), it took up more road width than horse-drawn vehicles. Letting cars loose on Central London's narrow streets was not really a viable long-term option.
In 1903 the Royal Commission on London Traffic was set up, and it reported in 1905. The need for road widening was stressed, and bye-laws recommended to control new development (preventing it from obstructing new road plans). The Commission recommended road widths of 140ft for 'main avenues' and 100 ft for 'First Class Arterial Roads'. A grid system (similar to American cities) was proposed, with two perpendicular major avenues: east-west linking Bayswater with Whitechapel via the City, and north-south linking Holloway to Elephant & Castle (both of these routes would also have trams on the road, and underground railways beneath).
W. Rees Jeffreys (of the Road Improvement Association, and the AA) took a different view: he saw ring roads, rather than a grid in the system, as a priority. He submitted a proposal to the Royal Commission for a "boulevard round London", doubtless influenced by similar roads in continental cities. His ring road follwed about the same route as the North and South Circular Roads do today.
Although the Commission had recommended the establishment of a Traffic Board for London, this was not set up until 1924. However, a new London Traffic Branch was formed, part of the Board of Trade, which surveyed the capacity of current roads, ways of increasing it, and the possibility of building new radial routes. They came up with 100 miles of new road schemes, including the North Circular Road, the Eastern (A12) and Western (A40) Avenues, among others, but did not favour the Royal Commission's grid-like dream. These proposals were announced in the General Road Plan, 1911.
The small matter of World War I prevented anything being done about traffic in London for a good few years, but in 1919 the Ministry of Transport was set up, and things began to start rolling. The planned roads were complete by the mid-1930s, and some more MoT-planned schemes were added too. However, the ambition of the plans was toned down somewhat: not many of the resulting new roads were dual carriageway, and most were under 30ft wide.