The reason nationwide emissions are as good as they are is because CARB is massively influential. Famously from a few years ago, Trump tried to make it illegal for states to set their own emissions standards because CARB's goals were much more aggressive than the auto manufacturers and other lobbyists preferred.

https://www.theverge.com/2019/9/18/20872304/trump-california...

They also were the first governing body to investigate VW's diesel 'defeat device';

https://www.nytimes.com/interactive/2017/business/volkswagen...