Figure 2. Tracing driver sources via the road usage network.

(a) The colour of a road segment represents its degree K_road. Most residential roads are found to have small K_road, whereas the backbone highways and the downtown arterial roads are shown to have large K_road. The light blue polygons and the light orange polygons pinpoint the MDS for Hickey Blvd and E Hamilton Ave respectively. The white lines show the links that connect the selected road segment and its MDS. The two road segments have a similar traffic flow V ~400 (vehicles/hour), however, Hickey Blvd only has 12 MDS located nearby, whereas E Hamilton Ave has 51 MDS, not only located in the vicinity of Campbell City, but also located in a few distant regions pinpointed by our methodology. (b) The degree distribution of driver sources can be approximated by a normal distribution with µ_source = 1,035.9 (1,017.7), σ_source = 792.2 (512.3), R² = 0.78 (0.91) for Bay Area (Boston Area). (c) The degree distribution of road segments is approximated by a log-normal distribution with µ_road = 3.71 (3.36) , σ_road = 0.82 (0.72), R² = 0.98 (0.99) for Bay Area (Boston Area). For more statistical analysis of the fits, see SI section III.B.