In 2019, London implemented something known as an Ultra-Low Emission Zone (or ULEZ). The intent was to reduce the number of older and higher-polluting vehicles entering and driving around the city.

It works like this: If you have a vehicle that does not meet the ULEZ emission standards, you need to pay a daily charge of £12.50. This applies all day every day (except Christmas) and it is in addition to London's congestion charge.

It's also done entirely through license plate cameras. If you enter the zone, don't have an approved plate, and don't pay the charge within a few days, you get sent a fine. The result is that London's ULEZ is now the largest clean air zone in the world (at least according to London).

It also achieved its intended purpose. In 2017, only 39% of cars entering London would have met the ULEZ emission standards. Today the number is over 95%. Meaning, most people don't actually pay the charge.

At the same time, nitrogen dioxide levels in the zone have more than halved, improving overall health outcomes. It's a perfect example of taxing the things you want less of. What's also interesting is that there were positive second-order consequences.

Vehicle traffic as a whole declined by about 9% in the first year, with no evidence of displacement to other areas. And according to this research study, it actually encouraged more kids to walk and take other forms of "active transport" to and from school.

Seems like a no brainer to me.