Public transport systems in Poland: from Bialystok to Zielona Gora by   bus and tram using universal statistics of complex networks
  We have examined a topology of 21 public transport networks in Poland. Our data exhibit several universal features in considered systems when they are analyzed from the point of view of evolving networks. Depending on the assumed definition of a network topology the degree distribution can follow a power law p(k) ~ k^(-\gamma) or can be described by an exponential function p(k) \~exp(-\alpha k). In the first case one observes that mean distances between two nodes are a linear function of logarithms of their degrees product. 
Julian Sienkiewicz and Janusz A. Holyst