Symmetricity and scaling of travel time estimates with distance. In Panel A, we present a scatter plot of estimated travel times from i to j versus times from j to i. It is seen that the estimates are rather symmetric, as they should. Note that each pair of cities corresponds to a single point in the plot, and the selection of which of the cities corresponds to i and which to j is made arbitrarily. In Panel B, we show how the estimates scale with distance. Overall, we can observe an approximately linear trend. There are a few points that correspond to clearly infeasible estimates due to erroneous data that seem to have very long travel times (500-800 minutes) even though the distance between city pairs is low (0-150 km).