Traffic restrictions applied in Barcelona are insufficient to comply with European air quality legislation

23 November 2021

A BSC study shows that the measures adopted to date must be accompanied by a drastic decrease in traffic to comply with the air quality standards imposed by the EU.

Without a significant reduction in traffic, the impact of measures such as superblocks or tactical urban planning has a rebound effect in neighboring areas.

Researchers from the Barcelona Supercomputing Center (BSC), in collaboration with researchers from the inLab-FIB laboratory of the Polytechnic University of Catalonia (UPC), have quantified the impact of the different measures that are being carried out to date to reduce pollution caused by traffic in the city of Barcelona. The study estimates that an optimistic renewal of the vehicle fleet and the 25% reduction in traffic expected by the city council by 2024 would reduce the concentration of nitrogen dioxide (NO2) by between 20% and 30%, which would hardly be enough to Comply with the annual average of 40 μg / m3 of NO2 legislated by the European Union. In addition, the World Health Organization (WHO) recently lowered the recommended NO2 limit for health from 40 to 10 μg / m3, posing an even greater challenge for the city.

Without this significant reduction in traffic, the impact of the Low Emission Zone (LEZ) and the implementation of measures such as “superblocks” or tactical urban planning is far from the objectives set by the European Commission. This is reflected in the study "To what extent the traffic restriction policies applied in Barcelona city can improve its air quality?", published in the scientific journal Science of The Total Environment.

The impact of superblocks, tactical urbanism and the Low Emission Zone (LEZ)

From the council it is expected that the application of these measures will help discourage the use of private transport to reduce it by 25% in the next three years (forecast that is included in the Urban Mobility Plan 2024). Under this assumption, the study has quantified the impact at street level of traffic restrictions under current demand and with the expected reduction of 25%. In particular, the researchers have calculated the impact of the measures adopted up to the time of the study or that are currently being carried out: reduction of 32 kilometers of vehicle lanes, creation of eight superblock areas and the implementation of the Low Emission Zone. For the latter, an optimistic hypothesis has been assumed that thanks to the LEZ all prohibited vehicles have been replaced by vehicles that comply with the Euro 6 emission standard - the cleanest -, which is not expected in the short term.

The results show that even considering a 25% reduction in traffic, the expected NO2 reductions would be between 20% and 30%, depending on the street. Which would leave the city at the limit of the levels set by the European Commission, and far from the maximum levels recently revised by the WHO. This shows that the measures adopted so far must be complemented with new traffic restrictions to improve Barcelona's air quality and keep the concentration of pollutants at acceptable values ​​for health.

Superblocks and tactical urbanism alone have a rebound effect

The study also shows that, when only measures related to reducing the space available for private transport are implemented, such as superblocks or tactical urban planning actions, changes in nitrogen oxide emissions (NOx = NO2 + NO) are negligible (+ 0.1%), since the measures have a rebound effect in neighboring areas. According to the article, these restrictions generate a redistribution of traffic along the network and therefore also a redistribution of emissions at street level, with variations in nitrogen oxides of up to ± 17% in specific streets as a result of the new vehicle routes and the variation of traffic flow and speed.

Goal: less than 40 micrograms of NO2 per cubic meter of air

Based on the previous limits established by the WHO, the European Commission established in the 2008 air quality directive that, to ensure the protection of human health, the average annual concentration of NO2 should not exceed 40 micrograms per cubic meter. The Barcelona City Council has detected that, after the stoppage caused by the confinement and despite the measures applied to limit road traffic, last October the concentrations exceeded this limit, as was already the case before the pandemic.

According to the results of this study, and considering that the WHO has reduced the recommended NO2 limit from 40 to 10 micrograms per cubic meter, Barcelona's air quality will continue to be unacceptable for the health of its citizens until there is a greater reduction of circulating traffic in the city.



Daniel Rodriguez-Rey, Marc Guevara, Mª. Paz Linares, Josep Casanovas, Jan M. Armengol, Jaime Benavides, Albert Soret, Oriol Jorba, Carles Tena, Carlos Pérez García-Pando


Simulation models used:

Different simulation models developed at the BSC and inLab have been used to carry out this study, specifically the models:

  • CALIOPE-Urban —a multiscale modeling framework developed at BSC, which combines the CALIOPE mesoscale air quality system (Baldasano et al., 2011; Pay et al., 2014b) with the street-scale urban dispersion model (Benavides et al., 2019)
  • The VML traffic simulator (Montero et al., 2018), developed at Inlab-FIB of the UPC.
  • The HERMESv3 atmospheric emissions model (Guevara et al., 2020) and the coupling and calibration with the VML simulator, in order to estimate the changes in traffic emissions induced as a consequence of changes in the transport network (Rodriguez-Rey et al. ., 2021) has been developed in the BSC