This makes it about as clear as it can be. To stay out of this pie chart, you need to keep a helmet on your head. Research has shown that helmets are extremely effective at preventing head injuries. Yet plenty of riders take their chances, and nearly all bike share services in the United States don’t offer helmets, as Lenny Bernstein points out in a piece I recommend reading.
There’s also a troubling cultural stigma around wearing a bike helmet.
But helmets have come a long way. There are foldable helmets you can easily carry around, and helmets made with aesthetics in mind.
Update: The post now links to a more current study on the safety of helmets. Thanks to alert reader Greg Billing.