FWIW, in Texas if you take the MSF (Motorcycle Safety Foundations) class, you get a permit that allows you to ride without a helmet. And if you have that permit, your passenger doesn't have to ride with a helmet, either.
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After posting that I looked into it more -- seems I was part right, part wrong, and now even more disgusted with Texas:
- According to the Texas Transportation Code Subtitle G ยง 661.003, you don't have to wear a helmet if you took the MSF (or if you have sufficient insurance). That's the part where I was right.
- But you have to be 21 years or older. That's the part where I was wrong -- so yes, a 5yo passenger w/o a helmet is illegal.
- However, the statute goes on to say "

h) An offense under this section is a misdemeanor punishable by a fine of not less than $10 or more than $50." This is the part where I'm disgusted with Texas. Although to be fair, the lawmakers were probably targeting 17yo kids and didn't foresee idiot parents taking kids on rides. There's always the chance that a cop might decide to slap a reckless endangerment charge on top. Next time I hobnob with the motorcycle cops who frequent the local coffee shop, I'll ask 'em their take on this.
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Having said that: as the parent of two elementary school kids and also as the owner of two motorcycles -- yeah, seeing that would have seriously creeped me out, too.
I dunno -- if you see that again, and you sincerely believe the child is in some danger, you could probably do worse than call 911 if you've got a cellphone. The worst the 911 operator could do is say "sorry, we can't do anything".
Jess