I say unusual in the sense of Japan seems to be the only one to have solved the issue of how do you transport 2 kids on a bike not with "big box" like the dutch and subsequently the entire western world but via 2 seats, they're apparently very popular over there as a means of transportation.
You keep coming back to the fact that people already have straps and airbags in the car, but car accidents still cause lots and lots of head injuries to people. So they should wear helmets, right? You could mitigate that. If not, why not?
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