In the interest of science I have conducted a few experiments over the last couple of years and have compiled the results .
Experiment number 1 riding a Longstaff trike conversion down hill and being ejected from the saddle and in to a lamp post at 20mph. Result multiple fractures of the shoulder blade and the top of the socket broken away requiring pinning, head injury's zero.
Experiment number 2 riding a bicycle at 25 mph and hitting subsidence whilst looking over your shoulder then being ejected over the bars. Result four broken ribs, a punctured lung, a fractured hip socket and spinal damage resulting in delayed onset Lhermitte's syndrome , head injury's small laceration to back of head .
Experiment number 3 riding a bicycle at approximately 12mph and being hit from behind by a car traveling at 30+ mph and being propelled 12-15 feet in to the air. Result laceration to back of head and laceration to elbow, head injury Subarachnoid haemorrhage (bleed on the brain).
Experiment number 4 riding a bicycle down hill at 33mph and losing traction on both wheels whilst cornering. Result a broken shoulder blade, 7 broken ribs 5 of them broken in two places resulting in a flail chest and a punctured lung with a bleed , head injury's small laceration to top of head .
Conclusion, no helmet was worn in any of these experiments and apart from number 3 no serious head injury occurred but I would argue that the forces involved in experiment number 3 would have far exceed the design parameters of a standard cycling helmet. To my mind body armour of some sort would have done more to mitigate serious injury in the majority of these experiments rather than a helmet which at best would have saved me a few stitches.
It's just yer head didn't hit anything. I can count 3 of 4 incidents where a helmet has stopped a scuffed head, and in one case concussion. The other was a broken spine and 4 ribs and my head went nowhere near anything.
Hit from side, OTB, head first into road, helmet damaged, slightly fuzzy head, broken ribs. OTB on MTB, helmet damaged and gouged on rocks, smashed up shoulder. Car hit from side, another OTB (bonnet this time) head slid down road but shoulder took impact.
A cut lip and thank goodness for a helmet is a bit silly.
At least when I broke my spine, I was most insistent I'd not hit my head and it was my lower back that was broken, as I lay motionless in the road - they listened once I showed them my lid, without a mark.