question: what are the reasons to believe that current design of cities is not sustainable?

Cities require a concentration of food, water, energy, and materials that nature cannot provide. In turn, a multitude of resources from long distances are necessary in order to meet the needs of inhabitants. This requires non-renewable inputs for transport and puts pressures on other systems that are being utilized to meet the needs/desires of urban inhabitants. Unfortunately, cities also produce huge quantities of waste, sewage and pollutants in the air and water. Clean-up and waste management is costly and the majority of the public is unwilling to live face-to-face with these unsightly byproducts [NIMBY]. This requires that the waste be shipped elsewhere, burned or put in landfills which involve more non-renewable inputs and causes further environmental contamination. Furthermore, most cities [particularly in the United States] were not designed for people; rather they are designed for cars. As we are close to or have already reached peak oil and the oil comes from far away and the costs are only going to rise. This makes the very infrastructure of cities unsustainable.

In order to address this issue and restructure cities to be more sustainable, cities should first be viewed as a functioning system instead of distinct parts. This can be accomplished by capitalizing on localized natural resources, such as using plants to heat and/or cool buildings, using recycled water, expanding/introducing public transportation, encouraging citizens to walk/bike, and planting food on rooftops and in abandoned/unused lots. Integrating cities into the local ecosystem [rather than imposing on them] also allows for a city to become more sustainable because it incorporates the natural environment upon which these urban settings are dependent upon.

Plan B, Chapter 6