  This paper addresses the issue of the formulation of weak solutions to
systems of nonlinear hyperbolic conservation laws as integral balance laws. The
basic idea is that the "meaningful objects" are the fluxes, evaluated across
domain boundaries over time intervals. The fundamental result in this treatment
is the regularity of the flux trace in the multi-dimensional setting. It
implies that a weak solution indeed satisfies the balance law. In fact, it is
shown that the flux is Lipschitz continuous with respect to suitable
perturbations of the boundary.
