1. Lecture 19 Modifications of the Basic Gravity Model
19-1 The Distance-Decay Parameter
2018/11/15
Jun Liang, UNC

2. 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
2018/11/15
Jun Liang, UNC

3. 19-1 The Distance-Decay Parameter (Cont.)
Example of Chicago’s Beta value change over time:
The decline in Chicago’s 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.
2018/11/15
Jun Liang, UNC

4. 19-1 The Distance-Decay Parameter (Cont.)
2018/11/15
Jun Liang, UNC

5. 19-1 The Distance-Decay Parameter (Cont.)
Changing parameters in spatial interaction models – Negative relationships VS. Positive relationships. See figure 7.24.
2018/11/15
Jun Liang, UNC

6. 19-1 The Distance-Decay Parameter (Cont.)
2018/11/15
Jun Liang, UNC

7. 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
2018/11/15
Jun Liang, UNC

8. 19-2 Complementarity: Attractive and Propulsive Forces
Distance
Variances for the
dependent variable:
Airline traffic,
Phone calls, etc.
Population
Functionalities
Complementairty
Other attractive forces
2018/11/15
Jun Liang, UNC

9. 19-2 Complementarity: Attractive and Propulsive Forces (Cont.)
Complementairty will increase interaction between destinations and origins. Example: Ullman’s 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.
2018/11/15
Jun Liang, UNC