A single badly maintained diesel car produces as much pollution as thousands of legal new cars.
Same goes for trucks -- the low in air pollution you felt was likely not due to less cars but less trucks (and possibly busses, depending on where in Europe you live)
A car without a catalytic converter generates as much toxic pollution as 100 new cars with catalytic converters. A car whose catalytic converter has broken and which has engine trouble produces even more. And diesel produces more than gas. (Though new vehicles of both types are about equal.)
Thus a single badly maintained diesel car can produce as much pollution as thousands of legal new cars. Smog truly is a situation where a tiny majority of vehicles produce the vast majority of the problem.
"Catalytic converters generate 100 times less toxic pollution. Installing catalytic converters into millions of cars prevented millions of tons of toxic pollution" gets turned into "A single badly maintained diesel car produces as much pollution as thousands of legal new cars."
Why turn a success into a failure? It only serves to make people feel powerless. Being "technically correct" was just a way to masquerade the real message "Don't try, you will fail and when you fail it will undo [0] your success".
[0] only a small part of (this is the fine print)
What the comment points to is that reducing car pollution isn't so much a question of saying we all need to pitch in as one of saying that we need to identify the serious problems and promptly deal with them. It is the kind of thing that is absolutely essential to understand before you have any chance of proposing an effective solution.
This is, for example, why it is common to have regular smog certifications required. And why those certifications do more to reduce smog than, say, telling everyone to drive 20% less would.
I understand it is not politically correct to say most cars are not the problem (or at least not the most of it).
In ideal world we would not be driving petrol guzzlers but instead have cars that run on electricity from our own home solar panels.
But the reality is that a well maintained car emits thousands times less harmful stuff than cars from couple decades ago. The new norms that automakers are facing are really crazy restrictive and sometimes hit the very limits of physics and thermodynamics.
The problem is when people get around these rules, drive unmaintained cars, drive cars that are way larger and more powerful than is needed or cheat by removing converters and installing them only for inspection.
Especially the last one. Here in Poland this is practically the norm and people treat this as part of life and feel proud they "saved". Then they complain we have bad air.
