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• The effect of lockdown on NO 2 in London, Milan and Paris was assessed. • Data from 52 air quality control units were studied. • Comparison with 2017−2019 highlighted a high decrease in NO 2 due to low traffic. • Despite low traffic, NO 2 concentrations > 40 μg m−3 were registered for several days. • Rethink the vehicles, the city and its vehicular traffic are strongly suggested. Nitrogen dioxide (NO 2) can have harmful effects on human health and can act as a precursor for the formation of other air pollutants in urban environment such as secondary PM 2.5 and ozone. The lockdown measures for CoViD-19 allowed to simulate on a large scale the massive and prolonged reduction of road traffic (the main source for NO 2 in urban environment). This work aims to selectively assess the maximum impact that total traffic blocking measures can have on NO 2. For this reason, three megacities (London, Milan and Paris) were chosen which had similar characteristics in terms of climatic conditions, population, policies of urban traffic management and lockdown measures. 52 air quality control units have been used to compare data measured in lockdown and in the same periods of previous years, highlighting a significant decrease in NO 2 concentration due to traffic (London: 71.1 % - 80.8 %; Milan: 8.6 % - 42.4 %; Paris: 65.7 % - 79.8 %). In 2020 the contribution of traffic in London, Milan and Paris dropped to 3.3 ± 1.3 μg m−3, 6.1 ± 0.8 μg m−3, and 13.4 ± 1.5 μg m−3, respectively. Despite the significant reduction in the NO 2 concentration, in UT stations average NO 2 concentrations higher than 40 μg m−3 were registered for several days. In order to reduce the pollution, the limitation of road traffic could be not enough, but a vision also aimed at rethink the vehicles and their polluting effects should be developed. [ABSTRACT FROM AUTHOR]