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That was my understanding.
However, I think one of the big issues for areas where the vehicles don't get up to decent speeds regularly is that all the various pollution reduction devices cease to function properly.
That's why things like ULEZs are important imo. So hopefully soon commercial vehicles in urban areas will all be electric and diesel will just be for motorways and fast A-roads.
Are modern diesels still really bad for PM2.5 and other emissions, euro 6 and DPF etc still not up to scratch? Is the greater MPG and stuff seen as insignificant compared to the air pollutants?