In the UK, Ireland and Europe, the petrol fuel nozzle is 21 mm in diameter and the diesel fuel nozzle is 25 mm diameter as standard EN 13012 so although it is hard to put a diesel nozzle into a petrol tank, the costlier and more damaging petrol into a diesel tank is easily managed.
Even when the nozzles have letters six inches high and colour coded jackets, under the sodium vapour lights of a forecourt at midnight, a tired or distracted driver can easily not register that they've selected the wrong nozzle as they are running on auto-pilot trying to get somewhere. The danger is the routine; you fill your Lamborghini with petrol twice a day so you forget you're driving you're wife's diesel Range Rover and you fill it with petrol.
Although there has been talk of redesigning fuel nozzles and filler caps, I haven't seen any measures in use. There are gadgets on the market which check your fuel nozzle is correct but they depend on the operator remembering to use them, which is obviously where the problem lies in the first place. There are some gadgets like this and this on the market that replace the filler cap with one that has a valve that can only be opened with the correct nozzle, the cheapest costs at least £40 and there have to be many variables made to fit around the huge variety of fuel filler cap and door designs. Perhaps the auto industry should fit them as standard but it seems the nozzles aren't all standard so you could find yourself unable to fill up with the right fuel anyway.
I have a simple fail-safe solution to the problem, one that is in use in the trucking industry in a similar vein and which presents a fantastic marketing opportunity for the first oil company to introduce it.
The oil companies should simply offer motorists a free sticker they can put on the fuel filler door or on the filler cap that contains a RFID chip which reports to a detector on the pump (with any kind of nozzle) whether it is attached to a diesel or petrol vehicle. If the wrong fuel for the tag detected is selected, an helpful alarm is triggered with a helpful "are you sure" override button. If the car doesn't have a tag, or it can't be detected, then the system lets the driver choose as normal. Many petrol stations already utilise similar systems for "pay at the pump" authorisation and fleet sales where RFID is proven technology.
If these tags were given away at petrol stations, the brand loyalty benefits would be enormous to the first company to introduce them.
After I had this idea myself, I googled it and found others are thinking exactly the same thing too. I can't understand why it wouldn't be a big hit with motorists and fleet operators and millions of these tags wouldn't be snapped up by grateful motorists.