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54 Validation of Atlanta, Georgia, Regional Commission Population Synthesizer John L. Bowman, Bowman Research and Consulting Guy Rousseau, Atlanta Regional Commission This paper presents the results of initial base- year and back- cast validation of the new Atlanta (Georgia) Regional Commission (ARC) population synthesizer (PopSyn), which acts as the conduit of land use infor- mation to the travel demand model. It takes informa- tion from the census and the land use model and creates a detailed synthetic population consistent with land use forecasts. A travel demand model can then predict travel for this population. The synthetic population includes a record for each household in the region and a record for each person in the household, so it is well suited for use by travel demand models employing disaggregate microsimulation. Although a PopSyn constitutes a pow- erful tool, it should be used with caution. By design, it provides misleadingly precise details about every person in the population. Because of limitations of its inputs and its synthesizing procedures, at best only some of the person and household characteristics accurately repre- sent the population at the regional level of geographic aggregation, and many of those characteristics can be imprecise and inaccurate for very small geographic areas such as census tracts. A fundamental goal in the devel- opment of a PopSyn therefore is to synthesize as accu- rately and precisely as possible, for as disaggregate geography as possible, as many variables as possible that determine travel behavior. And a fundamental requirement in the use of a PopSyn should be to rely on it only for the characteristics it accurately represents and to aggregate results to a level at which the synthetic population is precise and accurate. From the beginning, the Atlanta (Georgia) RegionalCommission (ARC) took seriously the need to use apopulation synthesizer (PopSyn) properly and insisted on being allowed to validate the synthetic popula- tion used for travel demand forecasts. Implicit in this insis- tence is the prerogative to adjust the synthesizer if the validation results are not as expected. With a flexible, adjustable PopSyn, validation can then become more of an iterative tuning procedure. The ARC PopSyn works in the following basic steps, common to many similar PopSyns. First, it starts with an estimate of the number of households in each zone, with details (in the cells of the matrix) for each of several demographic categories. It also has population forecasts for some aggregate categories. These control totals are more accurate but less detailed than the initial estimates. An iterative proportional fitting (IPF) procedure adjusts the detailed distribution to match the control totals. Then the adjusted numbers of households of each type are drawn from the Public Use Microdata Sample (PUMS). For the base- year population, ARC defined controls per transportation analysis zone (TAZ), with all the con- trol values coming from census tables [Summary File 1 (SF1), SF3, and Census Transportation Planning Pack- age]. The synthesizerâs design allows flexibility in the def- inition of the matrix cells and control categories so that a variety of one-, two- and three- dimensional census tables can be used to supply controls, thereby enabling the cap- ture of valuable joint distribution information available in the census tables. For families, the controls distinguish âwith childrenâ from âwithout.â For nonfamilies, the
