Figure 1. Distributions of traffic flow, betweenness centrality and VOC in the two urban areas.

(a) The one-hour traffic flow V follows a mixed exponential distribution for both Bay Area and Boston Area, where constants p_A and p_H are the fraction of arterial roads and the fraction of highways, v_A and v_H is the average traffic flow for arterial roads and highways respectively. (b) The distribution of road segment's betweenness centrality b_c is well approximated by , where the power-law distribution approximates arterial roads' b_c distribution and the exponential distribution approximates highways' b_c distribution. β_H denotes the average b_c of highways and α_A is the scaling exponent for the power-law. (c) The volume over capacity VOC follows an exponential distribution P(VOC) = γe^−VOC/γ with an average VOC = 0.28 for the two areas. Traffic flows in most road segments are well under their designed capacities, whereas a small number of congested segments are detected. For more statistical analysis of the fits, see the detailed discussion in SI section III.B.