Modeling Geographic Dispersion in an Urban Area
©2001 Nathan B. Forrester andMatthew S. Forrester
Background
• The Megapolis Project examined urban development patterns in Paris during 1990-1991
• The Megapolis model showed why a “multi-cellular” settlement pattern or distributed development poles make an urban area dysfunctional
Overview
• The Megapolis model simplifies and extends Urban Dynamics (Jay Forrester, 1969)
• Retained Concepts: – Jobs, Population, Business and Residential Structures– Attractiveness Principle
• New Concepts: – Geographical disaggregation– Commuting– Variable construction density
Structure
– Large area divided into a grid of 49 squares– Key Stocks in each area:
• Jobs• Residents• Business and Residential Structures
– Visibility of Jobs and Workers depends on commute time between areas
– Jobs, Residents, and Businesses change depending on visibility, occupancy, and density
Scenarios to Follow
• Initial conditions for Jobs, Residents, and Structures:– Balanced on average for the city– Spread evenly over the entire area
• Sequence of structural additions– Each scenario adds a new feedback concept to
the model
49 Stocks49 x 49 = 2401 Flows each for Jobs and Residents
Basic Structural Unit
People(by Area)
MovementOut
MovementIn
NetMigration
Attractiveness
Generic Urban Grid
Layout of numbered urban cells
29 13 5 1 3 11 2736 20 9 4 8 17 3344 25 19 12 18 24 4149 43 35 28 34 42 48
37 21 6 2 7 16 3245 22 14 10 15 23 4046 38 30 26 31 39 47
Scenario 1: Internal Movement due to Job/Worker Visibility
– Activate internal movement of residents and jobs between areas in response to visibility of jobs and workers
– Not yet activated:• Net Migration in/out of city• Construction• Occupancy effects on movement• Density effects on movement
Implosion
– Limited commute tolerance makes jobs/residents on opposite sides of the city invisible to each other
– High visibility of jobs and workers makes the center more attractive than the periphery
– Residents and businesses implode toward the city center
– Implosion ceases when most residents and jobs are all within an easy commute of each
People move to areas with higher job visibility
People(by Area)
MovementOut
MovementIn
NetMigration
Attractiveness
JobVisibiltity
-
400k 400k0
Jobs Residents
Scenario 1: Movement
Scenario 1: Movement
1
3
5
7S1
S2S3
S4S5
S6S7
0
100000
200000
300000
400000
Residents& Business
Scenario 2: Net In/Out Migration
• Structural Addition:– Positive feedback loop attracts people to the
city due to high job visibility, further increasing visibility
• Behavioral change: – Exponential growth in total population
proceeds in parallel with implosion to the city center
People(by Area)Net
Migration
Attractiveness
JobVisibiltity
City AvgAttractiveness to
ExteriorCity Job/LaborForce Balance
+
Attractiveness drives migration into and out of the city
1.2M 1.2M0
Jobs Residents
Scenario 2: Migration
Scenario 3: Occupancy Limits
• Structural Addition:– Occupancy rates of residential and business
structures impact attractiveness of an area• Behavioral change:
– Implosion stops abruptly as structures in the center become crowded and vacant structures on the periphery become attractive.
– Total population declines
People(by Area)
MovementOut
MovementIn
Attractiveness
Occupancy
Structures
-Net
Migration
People move in response to differential occupancy rates
400k 400k0
Jobs Residents
Scenario 3: Occupancy
Scenario 4: Construction
• Structural Addition:– Occupancy rates drives Net Construction of
residential and business Structures• Behavioral change:
– Implosion implosion and exponential growth dominate again as occupancy constraints relax
– Total population grows more slowly due to construction lags
People(by Area)
MovementOut
MovementIn
Attractiveness
Occupancy
Structures
NetConstruction
--
Occupancy drives construction
600k 600k0
Jobs Residents
Scenario 4: Construction
Scenario 5: Density Limits
• Structural Addition:– Density influences the attractiveness of an area
• Behavioral change: – Implosion and growth continue until the urban
core becomes sufficiently crowded to constrain business or residential growth
– Urban profile becomes flatter
People(by Area)
MovementOut
MovementIn
NetMigration
Attractiveness
Density
-
Density retards growth
400k 400k0
Jobs Residents
Scenario 5: Density
Scenario 6: Differential Density
• Structural Addition:– Businesses tolerate (or prefer) high density– Residents prefer living in low-density area
• Behavioral change: – Businesses crowd to the center– Residents spread out in a ring around the center– The city supports more people with lower
unemployment
400k 400k0
Jobs Residents
Scenario 6: Differential Density
Scenario 6: Differential Density
1
3
5
7
S1S2
S3S4
S5S6
S7
050000100000150000200000250000300000
Residents
1
3
5
7
S1S2
S3S4
S5S6
S7
050000100000150000200000250000300000
Scenario 6: Differential Density
Business
Policy Experiment 1: Improved Transit
• Parameter Change:– Time to travel a given distance drops
• Behavioral change: – The urban area expands horizontally– Population rises– In equilibrium, city is bigger, not better
Experiment 1: Transit
Populationv. Time:
-Base-Transit
PopulationProfile:
-Base-Transit
Experiment 1: Transit
Experiment 1: Transit
AttractivenessProfile:
-Base-Transit
Policy Experiment 2: Central Zoning Constraints
• Parameter Change:– Zoning restricts permissible density of business
construction density in core• Behavioral change:
– Businesses move out of the center– Most residents move further from the center– Some residents move back to urban core– Total unemployment rises
Experiment 2: Zoning
Unemploymentv. Time:
-Base-Transit
400k 400k0
Jobs Residents
Experiment 2: Zoning
Megapolis Simulator
• The Megapolis model is packaged in as a custom Venapp
• Interface was written using an automatic code generator developed in Microsoft Excel
• Order a copy on the sign-up sheet
Megapolis Simulator Dashboard
Venapp Code Generator
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