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They already do, but for another reason: The unleaded nozzle is narrower than the regular nozzle, so that you can't put leaded petrol in a car with a catalytic converter. This dates back to when all nozzles were the same, and leaded petrol was still widely available. The diesel pump retains its original (leaded petrol sized) nozzle.
So any physical device would have to rely on the car detecting the size of the nozzle or the nozzle detecting the size of the opening. The simple "It won't fit because it's too big" method has alread been used the other way around.
I thought that was the case. But wasnt sure.
A different shape might work, but it would require retro-fiting of all diesel pumps in advance of its introduction, otherwise people with new cars wouldn't be able to buy fuel.
I guess its been brainstormed many a time.
Currently in dave_mezza2006's garage:
2014 Vauxhall Astra Limited Edition 1.4 16v VVT Turbo