On a smaller scale in the town next mine lies a very large roundabout that I took as an urban design wart. Recently I accidentally learned [1] that it was built to fill the hole left for a national road exchanger. The national road project never completed due to public resistance.
The actual roundabout is average, but you can see an outer ring. Few years ago there were no buildings around it, it was raw grass. It was planned mid 60s, with the whole large scale urban projects craze. Here some original layouts http://imgur.com/a/ogD1V#0
A plan for a motorway round the centre of Edinburgh:
http://www.gcat.org.uk/blog/?tag=edinburgh-motorways
Churchill's Operation Unthinkable for an attack on the Soviet armies in Eastern Europe by Western forces in the aftermath of WW2:
http://en.wikipedia.org/wiki/Operation_Unthinkable