I've always wondered if we couldn't do an elevated bike trail in the San Francisco Bay Area. One of the challenges of doing an elevated freeway or road are the costs associated with supporting vehicles, but a huge number of bikes/pedestrians would not be quite as burdensome. On/off ramps would be a challenge as that is where they would most likely interfere with existing traffic patterns.

The Stevens Creek trail (an old rail bed in parts) is a good example of an urban trail. And of course highway 85 before they officially opened it, it was like four lane bike trail and exceptionally awesome.