UrbanSim takes several key inputs as exogenous. Two of these are from external model systems: a macroeconomic model to predict future macroeconomic conditions such as population and employment by sector, and a travel demand model system to predict travel conditions such as congested times and composite utilities of travel between each interchange. The latter is loosely coupled to UrbanSim, with land use predictions input to the external travel models, and travel conditions input to subsequent annual iterations of the UrbanSim land use model system.
UrbanSim operates on an annual scheduling of key model components, and data flow is as shown in Figure 2.1. The data store contains the current state of all objects in the system, with archiving as needed by individual model components, or as requested by the model user into ASCII extracts from the model. Each of the key model components are described in the following sections. The mathematical structure of the underlying procedures in the model are virtually identical for the household and employment aspects of the model system, so for brevity the household equations are omitted from the presentation below.
The model system reads exogenous inputs not only from external macroeconomic and travel demand models, but also from user input. These user inputs include assumptions reflecting land use policies that regulate real estate development, and any user-specified events that describe scheduled events representing changes in employment, real estate development or land policy the user intends to apply to the model in a simulation year beyond the initial, or base year.
The main model components, in the order of their execution, are the economic and demographic transition models, the household and employment mobility models, the accessibility model, the household and employment location choice models, the real estate development model, and the land price model. An export model writes simulation results in user-specified forms to output files for further analysis or processing, such as by travel demand models or by GIS.
Locations in the model are based on a grid with a resolution of 150 by 150 meters per grid cell. Cells are cross-referenced to Traffic Analysis Zone for indexing travel model outputs, and to city, county, and other geographic overlays for indexing land use policies that apply to specific jurisdictions or overlays.
