  In 2020 and 2021, the spread of COVID-19 was globally addressed by imposing
restrictions on the distance of individual travel. Recent literature has
uncovered a clear pattern in human mobility that underlies the complexity of
urban mobility: $r \cdot f$, the product of distance traveled $r$ and frequency
of return $f$ per user to a given location, is invariant across space. This
paper asks whether the invariant $r\cdot f$ also serves as a driver for
epidemic spread, so that the risk associated with human movement can be modeled
by a unifying variable $r\cdot f$. We use two large-scale datasets of
individual human mobility to show that there is in fact a simple relation
between $r$ and $f$ and both speed and spatial dispersion of disease spread.
This discovery could assist in modeling spread of disease and inform travel
policies in future epidemics -- based not only on travel distance $r$ but also
on frequency of return $f$.
