“A very low-speed fall can be just as dangerous as a fall at higher speeds,” said Randy Swart, director of the consumer-funded Bicycle Helmet Safety Institute. “All it takes is gravity – the distance to the ground – to cause a head injury.”

Teenagers seem especially resistant to wearing helmets, yet with their still-developing brains they probably incur the greatest risks and have the most to lose from a head injury. “There’s often a rebellion aspect among teens,” Mr. Swart said. “They say, ‘All through childhood, my parents forced me to be safe. Now I want to make my own decisions about risk.’” College students and young adults often also think similarly, he said.

I worry too about younger children, even those whose parents insist that they wear a helmet when riding a scooter, tricycle or bicycle. I see many such riders with parents in tow in my Brooklyn neighborhood, and in at least half the cases I’ve observed, the helmet is too big or is not on correctly and likely to provide little protection in a serious fall or crash.

The most common error is positioning: If the helmet sits too far back on the child’s (or adult’s) head, it will not protect the most vulnerable part of the brain in a hard fall, especially if the skull fractures. When the straps are too loose (or, as I’ve often seen even among adults, the chin strap is not clasped), the helmet will fly off in a fall and offer no protection whatsoever.

The helmet should sit on the head straight, front to back, and not move when you shake your head. The straps extending from the helmet to the chin strap should each form a V right under the ears.

“A bike helmet is a like a seatbelt – it should feel snug, not tight, when you first put it on, but when you start riding, you should be able to forget all about it,” Mr. Swart said.