When London's Ultra Low Emission Zone was first introduced in 2019, it was only 21 square kilometres in size, covering only the most central part of the city.
Today, after several expansions, the ULEZ covers an area of around 1,500 square kilometres, spanning all 32 London boroughs and encompassing the homes of more than nine million people.
The policy, which mandates that motorists with older, more polluting vehicles must pay a charge of £12.50 (US$16.71) to drive in the zone, was and continues to be a fairly radical policy.
While low-emission zones can be found elsewhere in Europe, never has one of such scale or standard been implemented.
It was introduced by Mayor of London Sadiq Khan as a public health measure after several shocking reports were released showing that poor air quality had played a contributing role in several deaths across the capital.
