An amazing project in Boston nicknamed the 'Big Dig' completed in December 2007 (9 years over schedule!) put Boston's Central Artery - Interstate 93 - underground. The old elevated freeway was removed and the space turned into Rose Kennedy Greenway, a series of parks...
The project had many major problems: It went way over schedule, over budget, 4 people died (partly due to dodgy material use). But, done properly, I can't help but feel this is such a beautiful idea that a previous mass of concrete road can be turned into an urban oasis.
It makes me wonder if we could put all roads underground? However it most likely wouldn't be safe and the experience of driving would be ruined.
I heard about this project whilst in Boston and even took the underground I-93 to the airport and back. Unfortunately I didn't get to experience the parks above for myself but here are some pictures I found: