Q-Free has improved the air quality in Sweden with its systems for the Congestion Tax projects in Stockholm (2006) and Gothenburg (2013). Q-Free systems identify eligible vehicles in the cities using video technology, and the corresponding tax is levied depending on the time of day.
Research has verified that both systems contributed to reduced levels of congestion and better air quality. Since the launch of the Stockholm system the number of passages within the congestion zone has been reduced by approximately 20%, carbon dioxide emissions have gone down by over 3%, air-borne pollutants are down around 13% and nitrogen oxides (NOx) has been reduced by around 9%.
A report from 2019 declared that congestion taxing has contributed greatly to improved air quality in Stockholm.