Here, I think, are some broad principles that weigh on the answer:
- A sustainable city has energy, water, transport, and waste infrastructure that is efficient and accessible, and that can be managed with minimal ecological impact.
- A sustainable city is a city conceived, designed, and managed so that its resources are efficiently accessible to all.
- A sustainable city uses research and outreach to make the use of sustainable practices and technologies a part of its mainstream culture.
- A sustainable city uses incentives, particularly tax and land use incentives, to direct investment in sustainable development and technologies and to provide stakeholders and citizens with motivation to change their behavior.
- A sustainable city has a municipal government that embraces and advocates for sustainability, and that is willing to invest monetary and political capital in people, programs, and technology in the move toward sustainability.
- A sustainable city has a population wants to be sustainable, and that wants to make the changes needed to be sustainable.