  We prove a quadratic interaction estimate for approximate solutions to scalar
conservation laws obtained by the wavefront tracking approximation or the Glimm
scheme. This quadratic estimate has been used in the literature to prove the
convergence rate of the Glimm scheme. The proof is based on the introduction of
a quadratic functional $\mathfrak Q(t)$, decreasing at every interaction, and
such that its total variation in time is bounded. Differently from other
interaction potentials present in the literature, the form of this functional
is the natural extension of the original Glimm functional, and coincides with
it in the genuinely nonlinear case.
