This paper presents a regional travel forecasting model system called SACSIM, used by the Sacramento (California) Area Council of Governments (SACOG) for planning and air quality analysis. SACSIM includes an integrated econometric microsimulation of personal activities and travel with a disaggregate treatment of activity purpose, time and location. Figure 1 shows the major SACSIM components. The Population Synthesizer creates a synthetic population, comprised of households drawn from the US Census Public Use Microdata Sample (PUMS~*) and allocated to parcels. Long-term choices (work location, school location and auto ownership) are simulated for all members of the population. The Person Day Activity and Travel Simulator then creates a one-day activity and travel schedule for each person in the population, including a list of their tours and the trips on each tour. These components, together called DAYSIM and implemented in a single custom software program, consist of a hierarchy of multinomial logit and nested logit models. The models within DAYSIM are connected by adherence to an assumed conditional hierarchy, and by the use of accessibility logsums. The trips predicted by DAYSIM are aggregated into matrices and combined with predicted trips for special generators, external trips and commercial traffic into time- and mode-specific trip matrices. The 'network traffic assignment models load the trips onto the network. Traffic assignment is iteratively equilibrated with DAYSIM and the other demand models.
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