Can't remember if I've mentioned this idea before. Apologies if I have.
Most of our most disgusting, polluting and selfish behaviour doesn't look that damaging at first glance. It's easy to ignore. This is certainly the case with motor vehicles. The roads are packed with unnecessarily massive vehicles powered by inefficient, oversized engines designed solely to make inadequate men and women feel important. They pump out huge amounts of poisonous fumes and lead directly to illness and early deaths.
But of course the driver thinks that he or she is unaffected. The air conditioning is on, and there's no obvious sign of the fumes. My first solution to this would probably be impractical: route just enough of the exhaust back into the vehicle's cabin so that the driver and passengers are left with impaired breathing and skin that permanently reeks of burning fuel.
As a poor second choice, how's this? Add a harmless coloured and fragranced dye to the exhaust outlet. A 4x4 would pump out thick clouds of black smoke, stinking of toxic substances. Or perhaps dog shit. A mid-efficiency car would produce greyer, slightly less voluminous clouds flavoured with brussels sprouts, chip fat, kippers or something equally unpleasant. Highly efficient cars would produce gentle blue or pink clouds fragranced with pine or apple scent. Emissions-free cars would merely exude an air of smugness. And perhaps recorded applause.
This way, drivers would be forced to see the consequences of their behaviour in an immediate fashion, and we'd all know which people and vehicles to avoid. It would work because most of the beautiful cars are also the most poisonous - Rolls, Ferrari, Range Rover, Porsche, Aston Martin. If we add smoke and stench, the drivers are forced to confront the hypocrisy of cocooning themselves in luxury while making others suffer.
(Another road-safety plan I have is to make anyone with points on their licences display large stickers of small children in crosshairs, like fighter pilot aces had tallies on their planes. One for each point as a public service warning). And Jeremy Clarkson should just have a photo of him on the bonnet so we all know he's around.