In my opinion, a sustainable city would make it easier for the average citizen living there to make greener choices and reduce their carbon footprint every day.  For example, we could install more bike lanes on the roads, along with more forms of transit going to more parts of the city more often.  Another change that I think is very important for lowering all cities carbon emissions is by implementing a carbon tax.  This means that each business, building, and household could set a reasonable goal of how much they want to limit their emissions to.  The city could track each of these buildings over the course of a year, and if the household or business exceeds their goal, they will be forced to pay a fee.  This would help hold businesses and all citizens accountable for their actions and provide an incentive to reduce their carbon footprint.