There's no point putting a helmet on if you don't do the strap up.

Many years ago, I was following a guy whose helmet strap was blowing in the breeze.
He got hit by a car turning across his path, his helmet was the first to leave his body, followed by his shoes.
The guy bounced a couple of times before smashing into the back of a stationary car, then his bike smashed into him for good measure.
After me and a buddy lifted the bike off him, we noticed he had "headbutted" the exhaust pipe of the parked car.
He had cut a channel from just above his eyebrows to the top of his head, about 6 inches long right down to his brain.
The skin, bone and hair remained in the exhaust pipe after he was taken to hospital.
Apart from the head injury, he broke virtually every bone down the right side of his body and survived, just.
That was back in 1982, at that time I used to campaign for bikers rights to have a choice on helmet wear. I never attended a rally after that.
A helmet is useless if not done up.