You need to do both by changing the building codes/zoning laws/public infrastructure best practice documents (think the things used to guide road design, etc).

You need to do both by changing the building codes/zoning laws/public infrastructure best practice documents (think the things used to guide road design, etc). Then all greenfield construction ends up looking much more like the awesome Netherlands and every time anything is built, or public infrastructure is renewed in a brownfield area it is brought up to the standard. It didn't take that many decades for the Netherlands to go from terrible to awesome using this method.