I think you are missing my point in asking for the proof that will “end the debate forever.” The fact the argument has been banging on for 20 odd years is the proof that the benefits of wearing a helmet are far from clear cut. If the benefits were clear there would not be an argument, at the very least there would be much less of an argument. Presently 20 years or so on there is no less of an argument than there ever has been. The onus of proof lies with the pro lobby and they have struggled with it for as long as internet forums have existed.
Bompington, Evidence? It's broadly in two parts. My first four whacks had me always wearing my helmet and I always hit my head. Two of these falls were quite innocuous and at quite low speeds but I always whacked my head when I fell with a helmet. My next four had me without a helmet and they were much more severe impacts. In two of them I didn't hit my head. But as I said these eight were ones where I broke bones, I came off many times otherwise and did not hit my head when I was not wearing a helmet and I always hit my head when I was.
The last of the whacks I had wearing a helmet involved a pedestrian on the A26 in Southborough, Kent, it's between Tonbridge and Tunbridge Wells. The pedestrian wasn't looking in my direction and stepped in front of me, there was no way of avoiding him and at the last moment I dipped my helmeted head to take the impact on my helmet. I glanced off one of his shoulders and weight forward ploughed headfirst into the ground. I was quite severely injured, face and left shoulder mostly. Looking back I realised if I had not been wearing said helmet I would not have led with my head and my weight would not have been so far forward: Much less likely that I'd have hit the road head first.
Whacks without a helmet: One was at the junction of Wormwood Street and Bishopsgate, the junction where a courier died last month. I was heading north at about 25mph when a driver, on his phone, before this was illegal, turned right off Bishopsgate into Wormwood Street. I did not have time to so much as touch the brakes, my front wheel hit the car just behind the front wheel arch sliding down the body work before fully engaging with the rear wheel arch, the bike with a Reynolds 853 steel frame folded in two and I got about 30 feet of air, landing beyond the junction. I did not hit my head.
Evidence II: A friend of mine was wiped out by an r sole in a van on Commercial Street. Like mine on Bishopsgate she got plenty of air. The driver was nicked and his insurers put up the usual dick head adversarial defence. An early question from the insurer's legal team was were you wearing a helmet. Her whack came some time after mine and by now helmets could apparently influence the claim through the BS of contributory negligence. In this case despite there being no head injury the insurer's lawyers keep banging on with the helmet question. After dealing with two half baked expert witnesses appointed by her lawyers - who had no real reply to the irrelevant helmet question - the victim asked the consultant who was treating her, Mr Tom Crisp, if he would offer his expert opinion. I believe the insurers lawyers were attempting to make the case if she did not hit her head the impact cannot have been as severe as claimed and therefore she was a malingerer and exaggerator. Of course they could not bring themselves to be completely clear with the accusation. Best to imply.
Tom Crisp is expert, he has been an orthopaedic consultant to the British Para Olympic squad, but he does not tart himself as a forensic expert. His evidence, which was accepted, was that helmets are more likely to cause an impact to the head because their mass and momentum, and its centre in an impact, will tend to overcome the hard wired muscular reflexes in most of us that will otherwise pull our heads out of the way of an impending impact as best as they can. These reflexes develop in proportion to the un-helmeted everyday mass of our heads. That might be opinion but it is certainly the case that an average helmet probably adds 5-10% in mass to an average head, and adds it to the head's extremity. Given the accelerations and decelerations we are talking of in impacts like falls I doubt anyone here can say said 5-10% extra would be controllable for necks other than those connected to the shoulders of F1 drivers and front row forwards.
In a cycling context - like helmets and jumping traffic lights, (see the links in the text that describes this t shirt): http://weadmire.net/tshirt/traffic-lights-t-shirt
– the risks we take are not as clear cut as people like to think.