  The dynamic spatial redistribution of individuals is a key driving force of
various spatiotemporal phenomena on geographical scales. It can synchronise
populations of interacting species, stabilise them, and diversify gene pools
[1-3]. Human travelling, e.g. is responsible for the geographical spread of
human infectious disease [4-9]. In the light of increasing international trade,
intensified human mobility and an imminent influenza A epidemic [10] the
knowledge of dynamical and statistical properties of human travel is thus of
fundamental importance. Despite its crucial role, a quantitative assessment of
these properties on geographical scales remains elusive and the assumption that
humans disperse diffusively still prevails in models. Here we report on a solid
and quantitative assessment of human travelling statistics by analysing the
circulation of bank notes in the United States. Based on a comprehensive
dataset of over a million individual displacements we find that dispersal is
anomalous in two ways. First, the distribution of travelling distances decays
as a power law, indicating that trajectories of bank notes are reminiscent of
scale free random walks known as Levy flights. Secondly, the probability of
remaining in a small, spatially confined region for a time T is dominated by
algebraically long tails which attenuate the superdiffusive spread. We show
that human travelling behaviour can be described mathematically on many
spatiotemporal scales by a two parameter continuous time random walk model to a
surprising accuracy and conclude that human travel on geographical scales is an
ambivalent effectively superdiffusive process.
