This paper [0] suggests improvements in car emissions has played a big role in reducing emissions in European cities as a whole. Vehicle emissions of all kinds have fallen pretty dramatically across Europe [1], although this is total emissions for vehicles, so it includes policies to reduce driving as well as those to reduce each vehicle’s emissions. So overall trends toward more efficient cars are certainly part of the story. Given these images are between 2007, when emissions had already been falling, and 2024, I’m inclined to think the policies highlighted in the article played a significant role as well.

[0]: https://www.sciencedirect.com/science/article/pii/S259016211...

[1]: https://www.eea.europa.eu/en/analysis/indicators/emissions-o...