Figure 20From: Gravity law in the Chinese highway freight transportation networksTesting the modified gravity law with per capita GDP for the daily directed and undirected freight highway transportation networks. (a) The directed network on 15 January 2019. (b) The directed network on 15 January 2019. (c) The directed network on 4 February 2019. (d) The undirected network on 4 February 2019Back to article page