The Grauniad last week reported a petrol shortage in Germany: they've introduced E10 (10% ethanol) fuel and no one will buy it, so they've run out of "normal" E5. The article suggests that 7% of German cars won't run on E10. Their government is insistent that this will save the planet, presumably as an alternative to admitting that it's a means of subsidizing agribusiness while dressing it up in green.
Does that sound familiar? It should, because "dressing it up in green" was what was done with "lead-free".
The level of lead in the atmosphere has been dropping at exactly the same rate since about 1930, and the introduction of lead-free has made absolutely NO difference. (The DfT graphs showing a drop all started from its introduction. The slope however was the same as it always had been. Technically it's called a "gee-whiz" graph.)
It was done because to meet Californian emission standards, it was cheaper (for the motor industry) to fit catalytic converters than to design efficient, clean engines. Otherwise Hondas, which already met the standards (ironically the clean engines all had two C's in the model name: Accent, Civic, Accord etc), would have a monopoly in a very important market. But. Catalytic converters need lead-free if they are not to need renewal every 1000 miles. Go figure.