  We investigate the nature of friction in granular layers by means of
numerical simulation focusing on the critical slip distance, over which the
system relaxes to a new stationary state. Analyzing a transient process in
which the sliding velocity is instantaneously changed, we find that the
critical slip distance is proportional to the sliding velocity. We thus define
the relaxation time, which is independent of the sliding velocity. It is found
that the relaxation time is proportional to the layer thickness and inversely
proportional to the square root of the pressure. An evolution law for the
relaxation process is proposed, which does not contain any length constants
describing the surface geometry but the relaxation time of the bulk granular
matter. As a result, the critical slip distance is scaled with a typical length
scale of a system. It is proportional to the layer thickness in an
instantaneous velocity change experiment, whereas it is scaled with the total
slip distance in a spring-block system on granular layers.
