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Two Kinds of Gravitational Forces in Transport: An Analysis Using the Gravity Model

The gravity model is widely used to estimate movements of people and goods. We use the gravity model to explain characteristics of movement of Korean people between and within cities via various modes of transportation using data from all 250 municipalities. We use a log-linearized gravity model to run regression analyses of traffic at three spatial levels (or dimensions): the national level (all of Korea), metropolitan level (in the Seoul and Busan Metropolitan Transportation Areas (MTAs)), and city level (in Seoul and Busan). For these geographical areas, we use two variables to represent gravitational mass in the model – regional population and regional GDP per capita (RGDPPC). The results show better estimations with national-level data but the explanatory power of the model decreases as the spatial level decreases and population density increases when population alone is included in the model. Introducing regional GDP per capita at both the city and metropolitan levels substantially improves the model fit. The results imply that including RGDPPC and population as two variables in the gravity model accurately reflects traffic flows in similar economically integrated areas, similar to how two kinds of mass create gravitational fields. With both these variables included, the gravity model can be effectively used to simulate traffic flows.