Interpreting Congestion Charge (Simple, step 1)

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Default-person Arthur Hjorth (Author)



Tagged by Didier Ruedin almost 10 years ago

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While at first sight, the purpose of the model may appear to be to test congestion charge as a policy, the underlying idea is illustrate how the same model may be interpreted differently by selecting and weighing some data over other.

The model illustrates the indeterminacy of the concept of public goods, based on Michael Feeden's (pp. 128-30, 2005) example.


Click 'Setup' to generate a population of 1000 with incomes ranging 1-100. By clicking 'Go' each person will decide whether to take the car or go by bus that day.

By sliding the 'Congestion charge cost' slider, you can increase or decrease the price of congestion charge.


The price sensitivity part of agents' decision is also simplified, and the amount of extra traffic per vehicle on the road is linear (when it should probably be exponential), etc.

Michael Freeden mentions the possibility of building more roads in his discussion of the example. This could also be a possible extension to the model.

Lastly, sliders or other ways for the user to manipulate these variables could be introduced:

avg-passengers-per-bus: 32

pollution-per-bus: 50

pollution-per-car: 7

avg-passengers-per-car: 1.2

population: 1000


This model was built by Arthur Hjorth ( and released under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 (

It is based on an example by Professor of Ideology Michael Freeden.

Freeden, M. What Should the "Political" in Political Theory Explore?. Journal of Political Philosophy, vol. 13 (2005), 113-134

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;; Interpreting Congestion Charge ;;
;; This model is based on Michael Freeden s example 
;; The purpose of this model is to show the ambiguity of political concepts such as public goods
;; and the interpretation of policies.
;; TODOS: price sensitivity 

globals [
  global-pollution-per-day               ;; pollution variable
  travel-time-added                      ;; traffic congestion, how congested is traffic. 
                                         ;; this variable adds itself to a vehicle s travel time
  average-travel-time                    ;; average travel time calculated per day per person
  yesterdays-travel-time-added           ;; the average travel time added per vehicle yesterday
                                         ;; this is key in people s decision making process for whether they want
                                         ;; to go by car or bus.
  turtles-driving                        ;; number of turtles driving on any day
  median-income-drivers                  ;; the median income of turtles who drive
  median-income-bussers                  ;; the median income of turtles who take the bus
  cars                                   ;; number of cars
  busses                                 ;; number of busses
  median-income-preference-met           ;; median income for those whose transport preference was met
  median-income-preference-not-met       ;; median income for those whose transport preference was not met
  turtles-preference-met                 ;; number of people whose prefernces are met
  turtles-preference-not-met             ;; number of people whose preferences are not met

turtles-own [
  income                                 ;; income level of turtle, 1-100
  car-love                               ;; how much the person wants to drive their car, 1-100
  willing-to-wait                        ;; how willing the person is to spend extra time on travelling by car
                                         ;; rather than travelling by public transport, 1-100
  wishes-to-drive?                       ;; would this person ideally drive today?
  can-afford-to-drive?                   ;; can this person afford to drive today
  will-drive-today?                      ;; will this person actually drive today
  preference-met?                        ;; true if person travels by preferred medians


to setup
  ;; clean up
  set avg-passengers-per-bus 32
  set pollution-per-bus 50
  set pollution-per-car 7
  set avg-passengers-per-car 1.2
  set population 1000

  ;; create population
  create-turtles population
    ;; setting shape person
    set shape "person"
    ;; placing randomly on map in unoccupied location
    move-to one-of patches with [any? turtles-here = false]
    ;;move-to first patches with  [any? turtles-here = false]

    ;; setting income randomly (will be changed as part of gini coefficient implementation)
    set income random 99
    set color white
    set car-love random 99
    set willing-to-wait random 99

    ;; adding 1 to all turtle properties to avoid divide by zero errors. Look up how to set a range for random and correct this. 
    set income income + 1
    set car-love car-love + 1
    set willing-to-wait willing-to-wait + 1

to go
  ;; switch over yesterdays time travel
  set yesterdays-travel-time-added travel-time-added
  ;; ask all turtles to decide on going by car or not
  ask turtles [decide]
  ;; calculate number of cars and busses
  ;; calculate the time delay created by number of cars
  ;; calculate median incomes for respectively drivers and non-drivers
  ;; calculate how many turtles traveled by their preferred medians
  ;; calculate median income for those who traveled by preferred and those who did not.


to calc-vehicles
  set cars count turtles with [will-drive-today? = true] / avg-passengers-per-car
  set busses count turtles with [will-drive-today? = false] / avg-passengers-per-bus

to calculate-median-incomes
  ifelse any? turtles with [will-drive-today? = true]
    set median-income-drivers median [income] of turtles with [will-drive-today? = true]
    set median-income-drivers 0
  ifelse any? turtles with [will-drive-today? = false]
    set median-income-bussers median [income] of turtles with [will-drive-today? = false]
    set median-income-bussers 0

to calculate-preference-met
  set turtles-preference-met (count turtles - count turtles with [wishes-to-drive? = true and will-drive-today? = false])
  set turtles-preference-not-met population - turtles-preference-met

to calculate-median-preference-met
  ;; if clause to avoid null pointer exc
  ifelse any? turtles with [(wishes-to-drive? = true and will-drive-today? = true) or wishes-to-drive? = false]
     set median-income-preference-met median [income] of turtles with 
           [(wishes-to-drive? = true and will-drive-today? = true) or wishes-to-drive? = false]
    set median-income-preference-met 0
  ;; if clause to avoid null pointer exc
  ifelse any? turtles with [wishes-to-drive? = true and will-drive-today? = false]
     set median-income-preference-not-met median [income] of turtles with [wishes-to-drive? = true and will-drive-today? = false]
    set median-income-preference-not-met 0
;to color-turtle
;  ;; coloring people per income
;  ;; dark red for lowest quartile, pink for next, light blue for next, dark blue for highest quartile
;  if income <= 25
;  [
;    set color 15
;  ]
;  if income > 25 and income <= 50
;  [
;    set color 17
;  ]
;  if income > 50 and income <= 75
;  [
;    set color 95
;  ]
;  if income > 75
;  [
;    set color 105
;  ]

to calc-pollution
  ;; calculates pollution by multiplying pollution per car with no of cars and ditto for busses
  set global-pollution-per-day cars * pollution-per-car + busses * pollution-per-bus

to calculate-delay-created
  ;; calculates delay created by finding sum of vehicles. All vehicles cause same amount of delay which probably isnt true...
  set travel-time-added (cars + busses)

to decide ;; turtle procedure. Decide on whether to drive that day or not.

to would-prefer-car ;; this determines if the turtle would ideally drive
  ifelse (car-love - (yesterdays-travel-time-added / willing-to-wait)) > 0
    set wishes-to-drive? true
    set wishes-to-drive? false

to can-afford-to-drive ;; this determines if the turtle can afford to drive. This needs to be tweaked to take into
                       ;; consideration feedback effect from congestion charge lowering price of public transp
  ifelse congestion-charge-price / 2 > income
    set can-afford-to-drive? false
    set can-afford-to-drive? true

to will-drive ;; this determines if the turle will actually drive
  ifelse wishes-to-drive? and can-afford-to-drive?
    set will-drive-today? true
    set shape "car"
    set will-drive-today? false
    set shape "bus"

There is only one version of this model, created about 13 years ago by Arthur Hjorth.

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