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i do find it a bit strange that i still seem to see plenty of vehicles that belch out black smoke, especially when the driver puts their foot down.
i'm sure the police have the authority to take a car off the road if it has a visible defect that would make it fail the MOT - they certainly do when it comes to bald tyres, or malfunctioning lights.
possibly it's just if it poses a danger, although you could argue that pollution is causing a danger to health...
maybe it's time for environmental police patrols?::edit::
https://www.gov.uk/report-smoky-vehiclelook like you can report busses or lorries, but not any other type of vehicle, to the DVSA
"The most significant area for diesel car owners is stricter rules on the permissible level of emissions of cars fitted with a DPF. If a car’s exhaust emits visible smoke of any colour, it will fail the MOT test."