Table 1.

Empirical results of the scaling exponent for two particular indicators of the cities in three different countries: one correlated to an infrastructure indicator (gas station) and other related to a socio-economic one (GDP). The data came from [8,29], but similar results were presented in other studies [9,10,29]. The number of gas stations scales sublinearly ($\beta \approx \frac{5}{6}<1$), while the GDP scales superlinearly ($\beta \approx \frac{7}{6}>1$) with the population size of the cities. Different countries present similar scale exponent values, and the evidence suggests a correlation between the exponents, given that the sum, no matter the country, is around 2.

country	βinfra (gas stations)	βse (GDP)	βinfra+βse
France	0.90 [0.8, 1.0]	1.20 [1.15, 1.26]	2.1 [1.95, 2.2]
Spain	0.75 [0.65, 0.85]	1.13 [0.97, 1.30]	1.88 [1.62, 2.15]
Germany	0.80 [0.75, 0.85]	1.17 [1.06, 1.28]	1.97 [1.81, 2.13]