Crain's New York Business has an article about how expensive it is to keep a car in New York City - not just Manhattan, but anywhere near the core (parking is probably a bit easier on the fringes but what do I know...). What is needed along with this is a concurrent and coherent strategy to push transit out into the further metropolitan region so that when city people stop driving they are not just replaced with people in cars from the outer counties of the region. Then you will have better air quality, reduced congestion and a more livable region because people are not so concerned about being killed by automobiles every time they leave their house.