As of 1 December 2017, Barcelona has limited the circulation of older vehicles within the city during air pollution episodes. To discover the true impact of the new legislation, the Real Automóvil Club de Cataluña (RACC), together with Barcelona Metropolitan Area (AMB) and Barcelona City Council, carried out the first field study to analyse motor vehicle circulation within the metropolitan area and its impact on air quality. The study measured emissions in real-life traffic situations from 92,365 vehicles at 31 points within the metropolitan area of Barcelona. The study found that 23% of the cars travelling within the metropolitan area will be affected by the new legislation, equivalent to more than 100,000 vehicles and a total of 250,000 journeys daily. In this context, and with the aim of making environmental improvement within cities compatible with the right of freedom of movement of the population, the RACC has presented a proposal to the public authorities with 14 measures for improving air quality. These measures are designed to address three broad aims: a progressive and uniform application of the restrictions on the movement of older vehicles, the establishing of financial incentives for users to replace their vehicle with one that is more environmentally friendly, and the improvement of infrastructure and the public transport system.