Lecture 19 Modifications of the Basic Gravity Model

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Lecture 19 Modifications of the Basic Gravity
Model
19-1 The Distance-Decay Parameter
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19-1 The Distance-Decay Parameter (Cont.)

• How to decided the distance exponent?
• Recall the example of Chicago phone calls, truck
  traffic, and air traffic.
• Variations in the distance exponent itself have
  become the subject of empirical study
• Different time (years, months, days, or weeks.)
• Different types of travel
• Different commodities shipped
• Different regions

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19-1 The Distance-Decay Parameter (Cont.)

• Example of Chicagos Beta value change over time
• The decline in Chicagos distance exponent
  represents the declining friction of distance in
  air transport as technological and economic
  changes that took place in latter part of the
  twentieth century.

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19-1 The Distance-Decay Parameter (Cont.)
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19-1 The Distance-Decay Parameter (Cont.)

• Changing parameters in spatial interaction models
  Negative relationships VS. Positive
  relationships. See figure 7.24.

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19-1 The Distance-Decay Parameter (Cont.)
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19-1 The Distance-Decay Parameter (Cont.)

• Surface transportation shows greater sensitivity
  to distance than does air transportation.
• Distance exponents for the truck flows (Chicago)
  are approximately 2.0
• British road and rail transport 2.5
• Air transport exponents are closer to 1.0
• The study of changes in the Chicago air exponent
  suggests a value of less than one for large
  gateway cities

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19-2 Complementarity Attractive and Propulsive
Forces
Distance
Variances for the dependent variable Airline
traffic, Phone calls, etc.
Population
Functionalities
Complementairty
Other attractive forces
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19-2 Complementarity Attractive and Propulsive
Forces (Cont.)

• Complementairty will increase interaction between
  destinations and origins. Example Ullmans
  agriculture surplus and deficit example.
• Basic Gravity model works for general cases, and
  it is hardly exist. It is important to look for
  other forces.