No need to be curt. He's simply saying that if Amsterdam is a much safer and convenient city for cyclists, one would expect both higher participation and lower

No need to be curt. He's simply saying that if Amsterdam is a much safer and convenient city for cyclists, one would expect both higher participation and lower incidence of injuries. A comparison of these metrics across these two cities would therefore be dominated by non-helmet-enforcement effects, and therefore it's more difficult to tease out the causal relationships than to simply compare the top level statistics.