In this study, we measured the NOx and O3 concentrations in different roadsides places, and one background site, in the city of Agadir, Southern Morocco, between January 2014 and June 2015. The observed concentrations were used to investigate whether the assumption that the photostationary state was reached, implicitly used in the air quality predictive models, was holding true. To our knowledge, this is the first time that this assumption is tested in roadside environments. Only in the background site, far from the direct traffic emissions, could the atmosphere considered in photostationary equilibrium. For the traffic sites, the excess NO from the tailpipe emissions does not have time to react or to be perfectly mixed with the background pollutants, leading to a significant deviation from the photostationary equilibrium. We suggest that the measured concentration ratio NO2/NO may be used as a criterion to evaluate the photostationary state of the urban atmosphere, which could be taken into account both for field measurements and for modeling purposes.