I would assume cycling safety increases proportionally with bike infrastructure. Something tells me the people dying on bicycles aren't dying from collisions with other bicyclists. If you have adequate infrastructure bike safety starts to become a marginal concern.
I grew up trained at school to always put on a helmet else I'd end up a vegetable after a bike accident. Not one mention of how American roads are designed for car supremacy at the expense of human safety. A helmet won't do you much good if a texting SUV driver hits you.
Motor vehicles aren't the only risk; whether a helmet would prevent serious injury in an accident depends on factors such as how fast you're travelling, and you don't need to be sliding along the ground too fast and connecting with a a kerb/a wall/street furniture for a head impact to cause life-changing injuries.
So I've been biking most of my life and I almost never wear a helmet. Not because I don't think they would help if I hit my head on the ground, but because in the past 20 years I haven't been injured while biking once. Maybe I'm just lucky, but I think 99% of it is just being cautious and aware of my surroundings while riding. At this point riding without a helmet is simply a risk I'm willing to accept.
