The present urban environment, especially in the U.S. tends to be designed for cars rather than human. The low density and high level of separation of different age group, different income group and different use also the separation from natural environment actually make the city a big prison where residents are trapped inside and have nowhere to escape to. When you live in a city where without a car you cannot anywhere (which is the case for lots of U.S. cities), when you are not old enough or too old to drive a car, then you are totally trapped. You can connect your friends through phones, facebook and other electric based methods, but you can never see them in person. The only thing you can do is to wait until your parents come back and willing to take you somewhere. This may be one of the reasons why people get bad impression on teenage drivers. After years of “jail" they are so desperate to rush their way somewhere else. If you are too old, you are thrown into special elder citizen residential facilities far away from normal social life. From then on you are never going to see the younger. People of high income live in their “Garden of Eden", completely separated from their low-income neighbors, thinking that they were all evil persons. There are always a certain percentage of good people and bad people in an each social group. Bias, scare and conflicts grow as a result of high level of separation and resulted lacking of contact.
These problems are believed to be associated with a poorly designed built environment and an un healthy (or “dysfunctional”) mode of development, commonly addressed as Urban Sprawl which has properties as: low density, rigidly separated land use, inaccessible huge blocks and an absence of vibrant city center (Reid et al., 2003) . A serious of multi-dimensional social, economic, environment and health problems are believed to be a result of Urban Sprawl as follows: higher driving distance, high car ownership, more polluted air, higher car accident death rate , less physical activity, high obesity rate, and high hypertension (Reid et al., 2003).