[semi-extrinsic]

A concrete solution which would have avoided "Dieselgate" is if the EPA had made urea NOx removal tech mandatory on all diesel vehicles. Saying "cars in the future must be X% cleaner" is needed, but it's not a complete solution.

Plus, more often than not, the technology to make cars cleaner is available, but expensive and so manufacturers don't use it.

Water-methanol injection, for instance, is well proven tech (since WW2), but not adopted at large scale because it requires changes to gas pump infrastructure etc. The only way to overcome a chicken-and-egg problem like that is governments making it mandatory.