PopulationSim

A travel model often needs a list of every household in a region and the people in it, with their ages, incomes and jobs.² Real lists of that kind are private. PopulationSim builds a synthetic one: an invented population that matches the published counts for each small area, so a model can hold "everyone" without holding anyone's records.² It began with the Oregon Department of Transportation, which wanted a shared, open tool for statewide, regional and city transport planning.^1,2 It is built on ActivitySim, the open activity-based travel model run by a consortium of metropolitan planning organisations and other transport agencies.^1,4

It needs two things. The first is a seed sample: real but anonymised household and person records, usually the US Census Public Use Microdata Sample (PUMS) or a travel survey, each with a starting weight.² The second is a set of controls: totals from the census or from forecasts, such as households by size or people by age, for areas like block groups, traffic analysis zones or counties.² PopulationSim adjusts the weights of the seed households until they add up to the controls at every level of geography at once, changing the starting weights as little as possible.² It then turns the weights into whole numbers and copies each sample household that many times.² The output is a household table and a person table for the region.²

The same machinery can reweight a household travel survey to match control totals, produce cross-tabulated counts for older four-step travel models, or adjust an existing regional population for a smaller study area.² It runs from the command line with a folder of configuration files and a folder of data, and the documentation includes a page on checking the results against the controls.^1,2

The result is only as good as its controls. Attributes that are not controlled are carried along from the sample households and are not matched to anything local.² The code is BSD-3-Clause and active; the latest GitHub release is v0.10.0 from November 2025, while the documentation site still describes version 0.5.1.^2,3,5