On my commute I regularly see riders cycling on quite busy roads with their helmets hanging from their bars - not on their heads.
Now we could have a debate on the the pros and cons of wearing a helmet but I'd sooner understand why they simply carry one. Do they wear it when they leave home just to keep the other half happy and remove it once around the corner to show how tough they are ? Why don't they just leave it home ?
Makes sense to me- you don't need to wear a helmet for it to protect you (or so I was told by various hospital staff, who said mine definitely saved my life when I had my big road crash, even though it was on a shelf at home 😉 )
Many moons ago, I once came off and injured my back and was told "that's why you should wear a helmet" - silly me, wasn't wearing one on my back. So maybe they're hanging their helmet there for when they ride home without any cushioning sandwiches in their rucksack, or something.
next time you see one of these people why not ask them if it works like an air bag and magically pops onto their head in the event of a crash?
Their mum makes them wear one so they ride round the corner with it on and then take it off.
I cycle the last km of my commute with my helmet on the bars when it's hot.