Western Futures
Appendix I. Our Land Use Modeling Methods
(extract from Report)
The Western Futures Growth Model (WFGM) projects future housing density as a main surrogate of overall growth and land development. It applies population estimates and a set of spatial rules to distribute future housing across the landscape. The model assumes that all land except federal or state lands (also minus some private land that is covered by water or on especially steep slopes) is open to development. Thus it does not "know" where land use tools like conservation easements, urban growth boundaries, or local open space programs have already protected land from development. The model produces scenarios of the future that can be compared to expectations or goals, and tested for the effects of land use planning.
The maps are based on 2000 US Census Bureau block-group and block mapping units. Current (2000) patterns of housing density were based directly on the block-level estimates of housing units. Historical patterns (prior to 2000) of housing density were based on block-group level estimates of the number of housing units, which were then spread to blocks based on the 2000 distribution. Population projections (which drive the housing density) were based on county-level population forecasts by state demographers where available, which we extended by adding an increment of population for each future decade equal to the population growth (total, not rate) in the last available decade of official estimates (because official estimates do not extend as far into the future as we wished, to 2040).
The model uses these basic rules in projecting future patterns: (1) Growth is computed as the average growth in each of four density classes (urban, suburban, exurban, and rural), and these are computed locally within a radius of about 1000 meters from the center of each census unit. (2) Growth estimates are then spread throughout the entire unit so that future growth is not constrained to occur where it had previously, though areas that grew in the past are first to get additional density. This approach means that future growth rates occur in a similar way as they have in the past, but since growth is parameterized locally, not within some artificial analytical unit like a state or county, different valleys or regions within a county can grow in a unique way. (3) The distribution of new growth is adjusted according to accessibility to the nearest urbanized area. That is, urbanization and conversion to suburban and exurban land use typically occurs in locations that are accessible to urban areas, but on the fringe, where land is undeveloped. Accessibility is computed as minutes of travel time from urban areas along the transportation network (major roads and highways. (4) The number of housing units is forced to meet the demands of the new population within a county. That is, the number of new units in a county is proportional to the number of additional people in a decade. Housing density is not allowed to decline over time, even in the few areas of the West that are not growing or are actually losing population.
A project of the Center of the American West
with funding from the William and Flora Hewlett Foundation
Western Futures
Contact: William Travis
Western Futures Team