Spatial ABM Dawn Parker, George Mason University SLUDGE model lab 25 Feb 2009

Spatial ABM Dawn Parker, George Mason University Exploring two sources of spatial complexity (transportation costs and neighborhood effects) using the SLUDGE model Questions to be explored: How does urban form change with: Transportation costs Market locations Positive spillovers between land uses Negative spillovers between land uses Initial urban land use patterns

Spatial ABM Dawn Parker, George Mason University SLUDGE (“Simulated land use dependent on edge-effect externalities) basic concepts SLUDGE is a simple abstract hybrid CA/ABM model Technical details to be covered later For today, we will use the model to develop our intuition about how transportation costs and neighborhood effects interact to influence urban land-use patterns

Spatial ABM Dawn Parker, George Mason University SLUDGE details In each time step, each cell (individually) is allocated to the most profitable of two land uses Cells in land use 0 are black Cells in land use 1 are white Each land use has: A market location Transportation costs to market Per-cell productivity

Spatial ABM Dawn Parker, George Mason University More SLUDGE details Land use zero has a fixed output price (set by the user) Land use one’s output price declines as more cells are converted to use one (important- what effect might this have? When might it be relevant?) Each cell changes use every other time period in a checkerboard pattern (“event sequencing”; will discuss later) Initial landscape can be all zero, or random

Spatial ABM Dawn Parker, George Mason University SLUDGE neighborhood effects Each land use can positive or negatively effect the productivity of its immediate neighbors Some examples? Thus, there are four “spatial externality” parameters to experiment with (0_with_0; 0_with_1; 1_with_0; 1_with_1) These should be set so that productivity is still positive

Spatial ABM Dawn Parker, George Mason University What is endogenous and exogenous? Endogenous: Land-use patterns (arrangement and quantity in each use) Market-clearing price for white land use Other output metrics derived from these two elements Exogenous Parameters set by the user in the “Sludge Settings, Parameters” panel

Spatial ABM Dawn Parker, George Mason University Transportation costs only: Replicating Von Thunen Positive transport costs for land use one Market one in the center of the landscape No externalities

Spatial ABM Dawn Parker, George Mason University Results Circular city form and downward sloping rent gradient

Spatial ABM Dawn Parker, George Mason University Decreasing transportation costs Same settings, lower transportation costs What changes do you expect?

Spatial ABM Dawn Parker, George Mason University Results, lower transportation costs Same form of city, expanded rent gradient flattens

Spatial ABM Dawn Parker, George Mason University Adding open-space amenities Keep all parameters as before, but add a positive value for white (urban) next to black How might you interpret this? What result do you expect?

Spatial ABM Dawn Parker, George Mason University Landscape pattern effects of open-space amenities Result is fragmentation, leapfrogging

Spatial ABM Dawn Parker, George Mason University Effects of initial conditions What if we start with an existing pattern of development? Interpretation is a change in policy and/or preferences from a given state (caveats)

Spatial ABM Dawn Parker, George Mason University Path dependence in urban systems Urban form and rents are no longer symmetric

Spatial ABM Dawn Parker, George Mason University Questions for you to investigate: When both land uses have markets/transport costs, how do the markets interact? Is white different? Why? How do different kinds of externalities affect landscape patterns? Do any externalities have observationally equivalent effects on patterns? On other outcomes? How much do outcomes vary with initial conditions with transport costs only? With externalities? Can you explain why?

Spatial ABM Dawn Parker, George Mason University Strengths and limitations of this simple model Strengths: Includes two generally important drivers of urban form Easy to learn and use, fast Supported by theory and empirical observation Limitations: Only one urban land use Euclidean transport costs Urbanization is not permanent Parcel sizes fixed

Spatial ABM Dawn Parker, George Mason University For more on the SLUDGE model: More detailed lab instructions: (seems to be gone!) Code: Documentation: Papers: Parker 1999 Parker and Meretsky 2004 Parker, Brown, Polhill, Manson, and Deadman 2008 (MR POTATOHEAD) Polhill, Parker, Brown, and Grimm 2008 (ODD)