Finish Ringway 2

London's ringways scheme devised in the 1960s saw a series of new motorways enveloping the capital, of which only one loop was ever completed - the M25, which is a combination of bits of the outermost ringways 3 and 4.

Ringway 2 was planned to go through the suburbs about half-way between the centre and the farthest reaches of the city, and was successfully built in the North and East, as the North Circular road. However, the South section was never built, making any vehicle journey through South London very difficult to this day.

Historically, South London has had many more overground railway lines compared with North London because the geology made tunnelling difficult. However, technology has progressed and this isn't much of a problem any more.

The many miles of existing railway corridors slicing up South London, combined with a lack of existing tunnels, gives rise to the possibility of rebuilding some of the railway lines underground using cheap, shallow cut-and-cover tunnels. A new four-lane (two in each direction) expressway could be built in the 16 - 20 metres of space freed up on the surface, thus completing ringway 2 with only a few demolished buildings, instead of the ~6000 that would have been cleared in the original plans.

Some of the neighbourhoods that the road would pass through are likely to be habited by influential NIMBYs who do not like the prospect of increased noise pollution. In such areas, the expressway and railways could swap over, with the road underground and the railway remaining on top. Of course, these neighbourhoods would have fewer access junctions to the tunnels meaning they would not be well served by the expressway, but as road haters I don't suppose they would mind.