Just in from my Okinawan informant:
Another interesting random fact about bikes in japan, it’s against the law to put your umbrella up while on your bike and it was controversial whether to ban umbrella holders (さすべえ or sasubee) as well. The thing is, umbrella holders are considered to be for old ladies (obasan) in tokyo, but in Osaka it’s difficult to find a woman without one- even young girls in their 20s!!!
They’re dangerous with the wind, umbrellas first of all, people are pretty determined to ride their bikes even in typhoons so they first banned using umbrellas, then i guess someone argued that having both hands on the bike doesnt really make it completely safe, they do fly away, like mary poppins I guess. And people try to ride their bikes in such crowded places, you know how it hurts when you get poked in the eye by one!
Check out more details and close up pictures of how these things work here (in Japanese).