It is to do with European Automotive Emission Standards
http://en.wikipedia.org/wiki/European_emission_standards

Where I live in Italy petrol cars that are not rated, or diesel Euro 3 or less and without exhaust particle filters, can be banned from circulation at certain times.

All the above are prohibited from entering the centre of Milan at any time unless they have special permission, and, on days of high pollution, usually in winter, vehicles with higher ratings may be banned, even in provincial towns.

Few other countries seem to have such stringent rules.

Peter

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