Table 2. This table displays the predicted theoretical behavior and the empirical observations versus the population size P for different quantities: L_tot is the daily total driven distance, A is the area of the city, L_N is the total length of the road network, δτ is the daily total delay due to congestion, Q_gas is the yearly total consumption of gasoline and is the total CO₂ emissions emitted yearly due to transportation. In the third (fourth) column, we show the predicted values of the exponent of P using the value of α (of a) measured on US data. In the fifth column, we show the value of the exponents directly measured on data about US and OECD cities. The measured values are in good agreement with the prediction. In particular, the exponents for L_N and δτ are consistent with our prediction that their difference should be 1/2.

		Predicted value of the exponents
Quantity	Theoretical dependence on P in self-consistant case	self-consistent case	fitting case	Measured value
		1	1	1.03 ± 0.03 (r2 = 0.95)
		2δ = 0.64	a = 0.85	0.853 ± 0.011 (r2 = 0.93)38,39,40
				0.765 ± 0.033 (r2 = 0.92)38,39,40
		1 + δ = 1.32	1.22	1.270 ± 0.067 (r2 = 0.97)38,39,40
		1 + δ = 1.32	1.22	1.262 ± 0.089 (r2 = 0.94)38,39,40
				1.212 ± 0.098 (r2 = 0.83)41
				1.33 ± 0.0326
		0.5	0.5	0.42 ± 0.02 (r2 = 0.83)38,39,40
		1 − δ = 0.68	1 − a/2 = 0.58	0.595 ± 0.026 (r2 = 0.90)38,39,40