Modelled NO_x concentrations are often underestimated compared to observations, and measurement studies show that reported NO_x emissions in urban areas are often too low when the contribution from traffic is largest. This modelling study quantifies the underestimation of traffic NO_x emissions in the Berlin–Brandenburg and finds that they are underestimated by ca. 50 % in the core urban area. More research is needed in order to more accurately understand real-world NO_x emissions from traffic.