Simple Agents, Complex Emergent City: Agent-Based Modeling of Intraurban Migration - Computational Approaches for Urban Environments - page 128

environment by virtue of changing spatial layers after taking actions such as building
new houses or moving between houses. The process of modeling TCMA intraurban
migration in the model has four steps: (1) establishing the spatiotemporal context,
(2) populating agents and environment, (3) running the model to create vacancies
and simulate migration, and (4) validating model output (Fig. 6.2 ).

6.3.2.1 Step 1: Spatiotemporal Context

We model intraurban migration in the seven-county TCMA from 2005 to 2007.
As an organizing framework, we adopt standard housing submarkets that map onto
well-established neighborhoods as defined by the regional real estate board (see
Fig. 6.3 ). We interpolate the population of each submarket as given by regional
government surveys and land-use zoning to fit in these submarkets as a series of
raster data layers with a resolution of 100 m.

6.3.2.2 Step 2: Agent Specification

The chief actors are households in the owner-occupied housing sector, housing de-
velopers, and governmental institutions. Populating actor agents involves significant
simplification, because the core strength of agent-based modeling is illustrating how
complex results can arise from simple actions. We focus on three types of agent.

Institutional Agents They shape housing development and migration destination
options. The model incorporates the policy effects of the regional planning agency,
the Metropolitan Council, and local governments through a set of areas that are
off limits to new housing (land reserved for agricultural use or wetland offsets)
and areas that are designated for new development (defined by growth zones and
sewerage availability). These effects are coded as rules that denote locations where
development can and cannot occur.

Developers They expand the housing stock and add new vacancies into the housing
market to join the existing vacancies given by the parcel dataset. Developers build
new houses that are added to the vacancy lists. Their key characteristic is the rate at
which they build houses, which is given empirically by the parcel dataset as 5,392
per year. They build houses at random locations in areas designated by institutional
agents. Using developers is a straightforward way of ensuring the growth in housing
mirrors that in reality while maintaining an analog to the real world, but their
decision making is far simpler than that of real developers.

Households These are the primary agent of interest. Agents are placed in the study
area via a polygon file where the number of households within each spatial unit is
determined by the actual household population in a given neighborhood, listed in the
population data noted above. The migration rate is around 7 % per year for owner-
occupied housing and this number of households is placed. Households are assigned