A big problem with city governments is they tend to worship economic growth.  They tend to be more responsive to business interests, ergo they protect the status quo, parking spaces, etc.  Getting councils to accept zero growth economies is probably the single biggest obstacle to sustainability in any city.  Tax and land use policies that favor local, walkable communities are usually at odds with merchants who want to increase their market exposure.

DK

Except local, walkable communities are actually better in a city like New York, where most people get around by public transportation. And taking up lots of space for parking is bad for growth, because parking just isn't a productive use of land. That's why all those parking lots and gas stations are getting replaced with buildings.
Turn it around.

Local governments tend to be dominated by NIMBYism.  For example, many outer suburbs of NYC have lobbied to be zoned exclusively for single family dwellings.

Metro governments may be for economic growth, but local forces are dead set against it.

I dunno DK. Merchants want to generate traffic to their stores. Most merchants in the the more dense walkable areas of the city rely almost exclusively on customers by foot, bus or subway. They benefit most when their area is seen as a desirable "Destination" for shoppers on foot. Enlightened self interest should drive local businesses to want their area to be a shopping destination.

For NYC this is OK. Moving more people into NYC is a huge environmental win regardless of anything else. NYCers don't (in general) have cars, and don't drive them much if they do. They have small to medium sized apartments surrounded on 5 sides by other apartments at roughly the same temperature.

Moving people into dense cities will do more for the environment than any sort of "green" work done on the suburbs.

In any case, realistically, just force all cabs (and busses) to be hybrids, ease zoning restrictions to get more housing built in the city, and try hard to expand the subways. That would be an excellent start.