There are no exhaust emissions at a local level, and that's it.
The areas with the poorest air quality tends to match population and vehicle density. Removing exhaust emissions at that level will be a significant factor on air quality where it matters most to individuals.
What about the causes of poor air quality from vehicles other than exhaust pipes?
What about the air quality away from where the car is being used and what about the other environmental impacts?
Finding alternatives to so many vehicles will have a much greater impact.
The areas with the poorest air quality tends to match population and vehicle density. Removing exhaust emissions at that level will be a significant factor on air quality where it matters most to individuals.