Force downtown and other high density areas to be automobile free.
Many dense cities force this upon their downtown areas.
Since transit / bike / walking options are good in high density areas, using cars is not needed.
The area can start small (like the areas directly around skytrain stations), and gradually increase once more infrastructure is there.
This will provide multiple solutions. Greenhouse gasses will be quelled, infrastructure costs will be lower, and most importantly, these area will become more appealing as public areas, which creates a higher sense of community and city identity.