Assumptions and Scenarios

This committee developed its own estimates of the potential savings in VMT, energy use, and CO2 emissions from more compact, mixed-use development, drawing on its review of the literature and the papers commissioned for this study. The committee’s estimates are focused on residential development patterns only.2 Two scenarios were developed relative to a base case. The base case assumes that current land use and travel patterns, which are heavily weighted toward suburban development and automobile-dependent travel, will continue into the future, producing a further decline in the overall average density of metropolitan areas, while the two alternative scenarios assume more compact, mixed-use development patterns.

Two forecasting periods are analyzed: the first to 2030 and the second to 2050. The starting point selected was 2000 because firm data exist on the number of households (from the U.S. census) and their travel patterns [VMT per household from the 2001 National Household Travel Survey (NHTS)] (Hu and Reuscher 2004). Uncertainties grow over time. For example, the 2050 estimates are less certain than the 2030 estimates because of uncertainties as to the numbers of house holds,3 their demographic and socioeconomic composition, and technological innovations that could change the nature of travel (e.g., extent of


The committee recognized the importance of commercial development, in particular that in employment subcenters that are readily accessible to housing. More compact development would presumably create more demand for commercial space to serve such developments. Nevertheless, addressing the uncertainties with regard to both the amount and the location of new commercial development (in existing or new employment subcenters or in strip development) and performing the modeling required to estimate potential reductions in VMT from improved access to commercial space were beyond the resources of this study.


For 2030 the range of household projections is relatively small. As projections are extended further into the future, a growing proportion of those who will be of household-forming age are yet to be born, and their numbers depend on future fertility rates. As a result, the uncertainties multiply and cumulate, and the range of the household projections becomes wider (Pitkin and Myers 2008).

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