Planning support systems and planning across scales: Comparing scenarios using multiple regional delineations and projections

Planning support systems (PSS) often employ urban models that simulate and evaluate impacts of plans. However, their application to plan making is, challenging when issues transcend local jurisdictions and model assumptions are contested by stakeholders. Neglecting the role of assumptions and specifications, especially when they are important and uncertain, can diminish the efficacy of plans. In this paper, we use the principles of scenario analysis to illustrate the impacts of two such important considerations-forecasts and regional boundaries-on model outcomes and related decisions. We use Montgomery County, Maryland, as a case and leverage a model developed for a larger region, i.e., the state of Maryland and vicinity. We develop two sets of scenarios-one where the county (a local government) freely competes with its neighboring jurisdictions for development and another where a higher-level (i.e., a regional or state) agency controls the extent of development that the county can receive. The scenarios are constructed using different specifications for regional boundaries and results in different amounts of growth in the county-both rare practices in scenario analysis with models. We then compare the outcomes on a set of indicators and draw implications for planning. We conclude with the argument that planning agencies should compare future scenarios, not just with different desirability, but also with different sets of assumptions and regional formulations.