Evaluation of the Effects of the Belt and Road Initiative on the Unified Economic and Environmental Efficiency of Transportation Infrastructure in China Based on Range-Adjusted Measure Model and Difference in Difference Model

Table 9

Estimated results of multiple mediation models for robustness test.

	Explained variable
Explanatory variable	Upgrade	Ration	UE
	(1)	(2)	(3)
	0.2726 (0.0218)	0.0209 (0.0117)	0.0117 (0.0041)
Upgrade			0.0157 (0.0089)
Ration			0.0387 (0.0119)
Constant	3.8871 (0.2640)	0.4059 (0.0688)	2.2814 (0.1398)
Control variable	Yes	Yes	Yes
Time fixed effect	Yes	Yes	Yes
Region fixed effect	Yes	Yes	Yes
Observations	360	360	360
	0.3086	0.6834	0.4722

Note: the standard error is shown in parentheses. , , and indicate, respectively, a significance level of 10%, 5%, and 1%.