The existence of lateral-superlattice effects in narrow In$x$Ga${1-x}$As/GaAs strained quantum wells grown on GaAs vicinal substrates is reported. The effects have been probed by photoluminescence excitation under magnetic field and compared to a theoretical model. Prior work indicates that strained epitaxial layers grown on vicinal surfaces may present a tilt angle between the substrate plane and the epilayer plane that depends on the lattice mismatch. This provokes strong lateral modulation of the potential induced by inhomogeneity of the built-in strain. Our results are consistent with strong lateral-superlattice effects.