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22 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 1 sizer. The population synthesizer provides flexibility in use gories. Age is controlled only in the version with 316 with current and future land use forecasts and it can be household demographic categories. Three age cate- adjusted based on validation results. gories are used. These categories are whether the head A population synthesizer is a powerful tool, but it of the household is over or under age 65, and for those should be used with caution. By design, a population households under age 65, whether any children under synthesizer may provide misleading details about every age 18 are present. In the forecast year, control is used individual in the population. A goal in the development only as the regional sizes of the subpopulations 65 of a population synthesizer is to synthesize, as accurately years of age and older and less than age 15. The fore- as possible, at as disaggregate a level as possible, with as cast control categories are sized by using relationships many variables as possible that determine travel behav- in the base-year census PUMS data between the avail- ior. A population synthesizer should be used for the char- able values and the control categories. Employment in acteristics that it accurately represents aggregated to a the base year is controlled at the TAZ level as the num- level at which it is precise and accurate. ber of workers in the household by four categories. For The ARC population synthesizer uses object- the forecast year, the control is used only for the region oriented Java software. It consists of subprograms called and only for the average number of workers per classes. Inputs include census data for the base-year and household. population forecasts for the forecast year. The population The results of the validation process indicate that synthesizer validates accuracy at multiple aggregation the use of census data to control for more variables in levels, including demographic and geographic levels. The the base year 316 version produces a more accurate results feed into the activity-based travel demand model. synthetic population. For the forecast year, the addi- The population synthesizer creates a synthetic pop- tional controls in the 128 and 316 versions provide lit- ulation for the base year and for each forecast year. A tle value. The results highlight a number of aspects public use microdata sample (PUMS) is used for the ini- related to the use of population synthesizers. The accu- tial distribution for the base year. For the forecast year, racy of the synthesized characteristics depends on the the distribution comes from the base-year distribution. control variables used for population synthesis, with The census tables are used for the controls for the base uncontrolled variables synthesized less accurately. year, while the controls for the forecast years use the land Accuracy also declines at more detailed levels of aggre- use forecasts. A synthetic population is produced for the gation. Accuracy for control variables can be influ- base year and the forecast year, along with a validation enced by rounding procedures and the use of averages. report that compares the synthetic population character- It is important to validate a population synthesizer by istics with known characteristics. The base year is 2000 examining these elements on a case-by-case basis. The and a back-cast to 1990 is used to validate the synthe- results of the validation process can be used to improve sizer's ability to generate a forecast population. the population synthesizer. Three versions of the population synthesizer were created for the initial testing and validation process. The simplest version of the population synthesizer has 52 MICROSIMULATION OF SINGLE-FAMILY household demographic categories. Two versions that RESIDENTIAL LAND USE FOR are more detailed have 128 and 316 household demo- MARKET EQUILIBRIUMS graphic categories, respectively. More detail from the census tables for the base year and from the ARC demo- Bin Zhou and Kara Kockelman graphic and land development forecast for the forecast year can be used as more categories are added. The com- Bin Zhou described a study examining single-family res- putation time lengthens as categories are added, how- idential developments for housing market equilibrium ever, and the increase in the number of sparsely using microeconomic theory and disaggregate spatial populated categories causes more rounding errors. The data. She discussed the use of a logit model and notions validation process helps identify the most appropriate of price competition to simulate household location household demographic categories to use. choices in Austin, Texas. She summarized the back- Household size is controlled at the travel analysis ground to the study, the data collection process, the zone (TAZ) level. In the base year, four categories are model of location choice, and the market equilibration used for the version with 52 household demographic results. Volume 2 includes a paper on this topic.3 The characteristics and five categories are used with the 128 following points were covered in her presentation. and 316 versions. Household size is controlled at the TAZ level in the forecast year, but only average house- 3See Zhou, B., and K. M. Kockelman. Microsimulation of Single- hold size is available. The base-year distribution is used Family Residential Land Use for Market Equilibriums. Volume 2, to translate this information into the controlled cate- pp. 6368.