The components of UrbanSim are models acting on the objects inFigure ![[*]](crossref.png)
, simulating the real-world actions of agents acting in the urban system. Developers construct new buildings or redevelop existing ones. Buildings are located on land parcels that have particular characteristics such as value, land use, slope, and other environmental characteristics. Governments set policies that regulate the use of land, through the imposition of land use plans, urban growth boundaries, environmental regulations, or through pricing policies such as development impact fees. Governments also build infrastructure, including transportation infrastructure, which interacts with the distribution of activities to generate patterns of accessibility at different locations that in turn influence the attractiveness of these sites for different consumers. Households have particular characteristics that may influence their preferences and demands for housing of different types at different locations. Businesses also have preferences that vary by industry and size of business (number of employees) for alternative building types and locations.
These urban actors and processes are implemented in model components that are connected through the software implementation shown in Figure 2.1. The diagram reflects the interaction between the land use and travel model systems, and between the land use model and the GIS used for data preparation and visualization.
UrbanSim predicts the evolution of these objects and their characteristics over time, using annual steps to predict the movement and location choices of businesses and households, the development activities of developers, and the impacts of governmental policies and infrastructure choices. The land use model is interfaced with a metropolitan travel model system to deal with the interactions of land use and transportation. Access to opportunities, such as employment or shopping, are measured by the travel time or cost of accessing these opportunities via all available modes of travel.
The data inputs and outputs for operating the UrbanSim model are shown in Table 2.2. Developing the input database is challenging, owing to its detailed data requirements. A GIS is required to manage and combine these data into a form usable by the model, and can also be used to visualize the model results. Once the database is compiled, the model equations must be calibrated and entered into the model. A final step before actual use of the model is a validation process that tests the operation of the model over time and makes adjustments to the dynamic components of the model. The steps of data preparation, model estimation, calibration and validation will be addressed in later chapters. In the balance of this chapter the design and specification of UrbanSim, using the most recent parcel-based approach used in the Puget Sound, is presented in more detail.
Employment data, in the form of geocoded business establishments