According to Wikipedia, exurbs lie beyond the suburbs of an urban area. A few miles of rural, wooded, or agricultural land typically separates exurbs from the suburbs. Note that:
"Many environmentalists, architects, and urban planners consider exurbs to be manifestations of poor or distorted planning. Extremely low densities - often featuring large lots and "McMansions" - create heavy car dependency (a very deliberate design choice). This also makes the construction of municipal infrastructure and deployment of services unusually costly and inefficient."Economic and tax policies that fuel the proliferation of exurbs thereby exacerbate the energy-dependence and environmental problems associated with automobiles. Lately, Austin municipal government officials have recognized the importance of density and have adopted policies that encourage urban over suburban and exurban development.