Localized and free excitons in GaN/Al0.17Ga0.83N quantum wells are studied by time-resolved photoluminescence (PL) versus temperature. We focus more particularly on 16-monolayer wide quantum wells for which, at T = 8 K, we observe double excitonic features. We assign the latter to two different localization states of excitons, from the temperature dependence of PL energies and of recombination dynamics. We discuss the reasons why double PL lines are observed in terms of the limitation of carrier in-plane mobility by interface roughness. We emphasize, in particular, the strong effect of the longitudinal electric field on the in-plane motion of carriers.