The West Side Elevated Highway was an elevated section of Route NY-9A running along the Hudson River in the New York City borough of Manhattan to the tip of the island. It was an elevated highway, one of the first urban freeways in the world, and served as a prototype for urban freeways elsewhere, including Boston's Central Artery.
When chunks of the highway's facade began to fall off due to lack of maintenance, and a truck and car fell through it at 14th Street in 1973, the highway was shut down, and a debate began whether to renovate it or dismantle it. Attitudes about urban planning had changed in the intervening decades, and the decision was made not to repair the decaying structure.
In the interim between the closure of the elevated highway and the completion of its dismantling, while debate about Westway was proceeding, remaining sections of the old highway structure began to be unofficially utilized as an elevated urban park, for jogging and bicycling.
(Photos via m-joedicke)