A London traffic policy has in just six months succeeded in sharply reducing toxic emissions and achieving better air quality in the city, according to city officials.

Mayor Sadiq Khan designated large portions of the fourth-most populous European city as an “Ultra Low Emission Zone” (ULEZ) last April, and revealed in an NPR interview on Tuesday that the plan’s implementation is directly linked to a 36 percent reduction in toxic nitrogen dioxide pollution.

Khan praised the results as “a huge improvement as we work to clean up toxic air and protect our children’s lungs.”

The ULEZ rule subjects drivers to a charge of £11.50 (almost $15) if they drive in the designated low-emission zone between the hours of 7:00 am and 7:00 pm. Daily fees are even higher for drivers with older cars that don’t meet current emissions standards.

The regulation has reduced daily traffic by more than 13,000 cars. The revenue from ULEZ fines is going toward improving London’s public transportation network, making the city’s 9 million residents even more likely to use public transit.

ULEZ has also reduced carbon emissions in the city by four percent, green transportation campaigner Kate Laing wrote.