I'll bring it back to being about helmets.
It seems like everybody wants to argue about helmets until they finally crash and hit their head on something hard enough to do damage, at which point they become converts.
My eighteen-year-old son, who goes everywhere by bike, has refused to wear a helmet, and we've had many discussions about it. "Not cool" and "too uncomfortable" and etc. etc. Whatever. On the way to a friend's house yesterday afternoon he hit some kind of uneven spot in the road and crashed his bike, and he says he remembers feeling his head hit the ground really hard.
I got the call that makes any parent's blood run cold: "Your son fell on his bike and hit his head and they're loading him into the ambulance." This was from an employee of the store in front of which he fell, who told me they'd keep his bike there for him.
He was very very lucky. He only got a deep gash in his head--nine stitches--and some road rash and a sprained hand and gravel embedded in his palm and lots of bruises. I'm glad and grateful that he wasn't hurt worse, but I'm also glad he had this opportunity to see firsthand the value of head protection.
If he'd had a helmet on, he'd still have been scraped up, but he would have gotten back on the bike and ridden home, or to his friend's house, instead of taking a ride in an ambulance and spending six hours in the ER.
He's a believer now, is going shopping for a helmet, and as soon as he can tolerate one on his head is eager to get back on the bike.
It's also worth noting that the kind of accident he had could happen anywhere--could happen in your driveway--so the arguments people make about not wearing a helmet "in the neighborhood" etc. really seem kind of silly to me. You can fall anywhere, for any number of completely unpredictable reasons. If they were all predictable, there'd be no such thing as accidents.
Fall from bike, land on head. That's all it takes. It only takes once.