Figure 2

Extraction of typical travel times between Senegalese cities. Panel A: Each set of cell towers assigned to a city are colored with same color. Black dots indicate cell towers that were not assigned to any city. Light blue background indicates sea, and white background land. Note also that southern and northern parts of Senegal are separated by Gambia, for which road data is not shown. Panels B and C: Two examples of inter-observation time distributions (B: from Kaolack to Tambacounda; C: from Dakar to Ziguinchor). The empirical inter-observation time distributions are shown with black dots, and the green curve represents our kernel density estimate of the inter-observation time probability density. The red vertical lines indicate the estimated typical travel time corresponding to the peak, and the blue vertical lines indicate the lower bound estimates. In Panel B, we see a typical pattern with a single clear peak that is located at 275 minutes (4 h 35 min). This gives us an estimate of the typical travel time from Kaolack to Tambacounda. In Panel C, however, there are two peaks. The first peak is located at ∼100 minutes and is presumably from air traffic between Dakar and Ziguinchor. The second peak is located at 870 minutes (= 14 h 30 min), which matches well with the travel time taken to reach Ziguinchor from Dakar with ferry (15 h).