[semi-extrinsic]

Cleaner air in LA is primarily due to two technological breakthroughs: positive crankcase ventilation, and catalytic converters. That's the proper way to do regulations: once a cleaner tech is invented, make it mandatory. The other way around, mandating a steady decrease de-coupled from technological advances, is more likely to lead to cheating.

[tremon]

But how do you direct the industry to invent cleaner tech without the pressure of emission regulation?

[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.