  The width W of the active region around an active moving wall in a directed
percolation process diverges at the percolation threshold p_c as W \simeq A
\epsilon^{-\nu_\parallel} \ln(\epsilon_0/\epsilon), with \epsilon=p_c-p,
\epsilon_0 a constant, and \nu_\parallel=1.734 the critical exponent of the
characteristic time needed to reach the stationary state \xi_\parallel \sim
\epsilon^{-\nu_\parallel}. The logarithmic factor arises from screening of
statistically independent needle shaped sub clusters in the active region.
Numerical data confirm this scaling behaviour.
