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TRANSIMS
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TRANSIMS FUNDAMENTALS

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ADDITIONAL TRANSIMS INFORMATION

TRANSIMS

Population Synthesizer

The Population Synthesizer module in TRANSIMS uses Census data to build synthetic households for the study area, and also uses land use data to locate the households relative to the transportation network. The output of the Population Synthesizer module is the synthetic households with a set of information associated with each household and each individual living in that household. It also provides the household location in the TRANSIMS network including the information on vehicles belonging to each household. The data input and output for the Population Synthesizer can be summarized as shown in the figure below.

Population Synthesizer: Input/Output flow for Population Synthesizer data. The chart shows four types of inputs; Network Data (activity locations), STF-3A (summary tables of demographics available at census tract and block levels of detail), PUMS (5% sample of census records, PUMA consisting of census tracts etc., approximately 5000 individuals) and TIGER/Line (using MABLE/Geocorr and geographic layout of census tracts and block groups). The inputs go into the Population Synthesizer and generate three types of outputs; Synthetic Households (location, census tract block group), Synthetic Persons (gender, age, schooling, employment type, location and hours, transportation and income) and Vehicles (vehicle ID, household, initial network location, vehicle type and emissions type).
Input/Output flow for Population Synthesizer data

Synthetic households are usually generated for a block group within a census tract, and each synthetic household is classified either as family, non-family or individuals living in group quarters such as dormitories. Each individual has an associated set of demographics which may consist of age, income, status, etc. These demographics are matched closely to the demographics of the real household. Using the census data of STF-3A and PUMS, the population synthesizer can estimate a proportion of real households for each block group in each demographic category using iterative proportional fitting (IPF). Synthetic households are drawn from the PUMS data for each block group according to these resulting proportions. TRANSIMS not only creates synthetic households but also matches the demographics between synthetic households and real households. This includes the household structure, the individual incomes and ages, those individuals who work and those who attend school, as shown below.

Final IPF Table for example Block Group

Householder’s Age (HHAGE) Total
Workers 15-24 25-34 35-44 45-54 55-64 65-74 >74
0 1 3 2 2 2 6 14 30
1 4 14 10 7 9 7 5 56
2 29 63 56 40 18 10 9 225
>2 0 2 4 4 3 1 0 14
Total 34 82 72 53 32 24 28 325


Corresponding proportions of family households for example Block Group

Householder’s Age (HHAGE) Total
Workers 15-24 25-34 35-44 45-54 55-64 65-74 >74
0 0.003 0.008 0.005 0.005 0.008 0.019 0.044 0.091
1 0.011 0.044 0.032 0.023 0.028 0.022 0.015 0.175
2 0.089 0.193 0.172 0.124 0.056 0.029 0.026 0.689
>2 0.001 0.006 0.011 0.013 0.009 0.003 0.001 0.044
Total 0.104 0.251 0.220 0.165 0.101 0.074 0.086 1.000


The PUMS data for the example Block Group 1

Householder’s Age (HHAGE)
Workers 15-24 25-34 35-44 45-54 55-64 65-74 >74 Total
0 3 12 14 11 21 51 54 166
1 9 47 63 44 59 45 14 281
2 18 56 113 83 45 17 6 338
>2 2 17 70 88 55 17 3 252
Total 32 132 260 226 180 130 77 1037

Use weighted sampling to get 63 households from the group of 56 in PUMS bin.

Example Household sampled from the PUMS cross class bin

  person running person with briefcase child riding bike
Age 26equation 26 7
Income $25K $18K $0
Status Head of Household Spouse Child
Work Status Worker Worker Hence: equation Student
Household Weight, wp 201

After the synthetic households are created, they are distributed spatially to approximate regional population distribution. The Population Synthesizer associates these households to activity locations on the walk link of the TRANSIMS network according to land-use characteristics associated with the activity locations on that link, as shown in the figure below.

The Population Synthesizer creating synthetic households and placing them on the network. This graphic shows PUMS data becomes synthetic population becomes households which are placed on the street network within a census block.
The Population Synthesizer creating synthetic households and placing them on the network

Then vehicle ownership is generated for each synthetic household as given by the PUMS data. All the outputs obtained from the Population Synthesizer are used as inputs for the Activity Generator module.