  We consider a hyperbolic system of conservation laws u_t + f(u)_x = 0 and
u(0,\cdot) = u_0, where each characteristic field is either linearly degenerate
or genuinely nonlinear. Under the assumption of coinciding shock and
rarefaction curves and the existence of a set of Riemann coordinates $w$, we
prove that there exists a semigroup of solutions $u(t) = \mathcal{S}_t u_0$,
defined on initial data $u_0 \in L^\infty$. The semigroup $\mathcal{S}$ is
continuous w.r.t. time and the initial data $u_0$ in the $L^1_{\text{loc}}$
topology. Moreover $\mathcal{S}$ is unique and its trajectories are obtained as
limits of wave front tracking approximations.
