A friend whom I met through work almost 30 years ago got into cycling about 10 years or so back, he's had two big offs, one in Spain (or one of the Islands), he was going downhill at speed, too fast for his abilities and hit a wall off the bend smashing his knee. I said to him if you'd not being wearing a helmet would you have been going slower, he said yeah, but the helmet might have saved me hitting my head ... except he didn't hit his head at all, ruined his holiday and couldn't work for some period afterwards.
The difference between rugby and gridiron, albeit it's smaller between union than league, is that you temper/control your aggression, the rules do not allow for head high tackles etc. Of course concussions/head injuries occur, I got a head injury last year putting in a big hit on a large guy who shouldn't have stuck his nut down to protect himself as I went to tackle him and my first ever head injury playing rugby!
, however the prevalence of reported concussions (many are not) within gridiron are massive, and the millions suffering CTE even if they didn't turn pro has a profound effect on so many, mostly males and has led to suicide.
The helmets simply did not prevent the micro TBIs despite the helmets being improved significantly and frequently over time.
The single biggest safety occurrences within the sport re head injuries was the change in the rules for 2018, lesser contact to the head in many aspects, both toward the QB but importantly receivers who were often the target of some shocking hits to the head using the helmet. This meant more care by the would be assailant ... sorry tackler
this ended up with a 45% reduction in (reported) concussions, it had abslutely zero to do with the helmet.
Changing the rules aka laws, for motorists, changing their behaviour and that of the person atop the bike regards risk taking and endangering others would be the two biggest things regards cycling safety. it's no coincidence that one of the biggest youtube cycling video makers commented that the modern pro rides much closer together than they ever did back in the 90s, the number of crashes/injuries and indeed deaths have gone up since helmet rules were enforced, despite all the other advantages of better brakes, better/grippier tyres,(arguably) better handling bikes, more on course protocols, more marshalls/signage/warnings etc.