Road traffic emissions are considered a major contributor to urban air pollution, but clean air actions have led to a huge reduction in emissions per vehicle. This raises a pressing question on the potential to further reduce road traffic emissions to improve air quality. Here, we analysed ~11 million real-world data to estimate the contribution of road traffic to roadside and urban concentrations for several major cities. Our results confirm that road traffic remains a dominant source of nitrogen dioxide and a significant source of primary coarse particulate matter in the European cities. However, it now represents a relatively small component of overall PM2.5 at urban background locations in cities with strong controls on traffic emissions (including European cities and Beijing) and many roadside sites will exceed the WHO guideline (10 μg m−3 annual mean) even when this source is eliminated. This suggests that further controls on traffic emissions, including the transition to a battery-electric fleet, are needed to reduce NO2 concentrations, but this will have limited benefit to reduce the concentration of fine particles, except in countries where the use of diesel particle filters is not mandatory. There are substantial differences between cities and the optimal solution will differ from one to another.