I would maybe just expand a little bit on Mr. Harcourt's answer.
I think it's correct. Getting a precise answer to this is hard. On the other hand, if you're asking where the biggest bang for the buck is likely to come from, it's going to be new mixed-used development, because of the fact that you're integrating your basic infrastructure into the buildings. If the buildings are already there and the infrastructure is already there, it's harder to get at.
I guess one of the reasons we've been pushing on this and saying we think it's urgent is that every day we're building new communities, and every day we're building them the way you just described. So there's lots to get at with new buildings. There are retrofit opportunities, especially with building complexes like universities and hospitals, where you have single owners and scale. The hardest thing to get at is the individual residential market.