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.
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.