The big decisions are not for the mayor to take. Many urban areas are in breach of an EU standard for the amount of particulates in Breathing Air.
We are not in the EU any more, why are we bothered? My guess (I don't really know) is that people with health problems like not being able to breathe will start suing the government. There was a coroners decision on that in South London.
So areas with the high levels need to have schemes. To deflect the heat from themselves, the government put the design of the schemes over to the local authorities. But, the government gets to refuse a scheme if it is either too expensive, or it clearly isn't going to have an impact on particulates.
The mayor had some schemes turned down for these reasons.
I guess the reason that Bristol is badly affected is that it is a bowl surrounded by hills, gorges and escarpments. If we are pumping particulates into the air at the bottom, that is where it is difficult to escape from and builds up.
Sorry, that was a bit boring.