We complete and unify the works by Oh and Zumbrun (2003a) and by the author (1994), about the spectral stability of traveling waves that are spatially periodic, in systems of n conservation laws. Our context is one-dimensional. These systems are of order larger than one, in general. For instance, they could be viscous approximations of first-order systems that are not everywhere hyperbolic. However, modelling considerations often lead to higher order terms, like capillarity in fluid dynamics; our framework remains valid in this more general setting. We make generic assumptions, saying in particular that the set of periodic traveling waves is a manifold of maximal dimension, under the restrictions given by the conserved quantities. The spectral stability of a periodic traveling wave is studied through Floquet's theory. Following Gardner (1993), we introduce an Evans function D(λ, θ), being λ the Laplace frequency and θ the phase shift. The large wavelength analysis is the description of the zero set of D around the origin. Our main result is that this zero set is described, at the leading order, by a characteristic equation This formula involves a flux F, which enters into a first-order system of conservation laws, describing the slow modulation of the periodic traveling waves. Its size N is in practice larger than n. The important consequence is that hyperbolicity of the latter system is a necessary condition for spectral stability of periodic traveling waves. Finally, we show that a similar treatment works for coupled map lattices obtained by discretizing systems of conservation laws.