I'm not too sure, the chart is percentage based, and although the ratio of 4 wheels to 2 is very high in somewhere like the USA, they also have the vehicles spread over a far larger area. I doubt if the city traffic densities are much different between many of those countries.Surely this reflects the number of 4 wheel vehicles on the roads, causing accidents? And in those cases a helmet would most probably not have had any life saving effect.
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