Same with London. Too easy to get caught even after researching before travel. A few years ago an Australian friend who had been staying with us, went from here to Gatwick in a hire car. Despite having a SatNav and I discussed the route with him before he left. We spoke when he got to Gatwick. He said he knew he had gone wrong when he drove over Westminster bridge! I told him he had gone through the zone. He said they would not be able to fine him in Australia.
I knew different. They could take it from the hire companies deposit. So I called the hire company, who gave me the reg number, and I paid the fee, £9 if I remember.
I agree, there should be standardisation.
I think it happens in France for cities that join their scene, I got an exemption sticker for my windscreen. Don’t know what glue is used but it is very perminant.
John