I keep getting into arguments about this on road.cc, but I think the infrastructure ship has sailed. There's barely any room in city centres as it is. Besides, I don't think infrastructure actually does anything to improve driver behaviour, and given most of my riding is done on roads that will never get dedicated infrastucture in my lifetime. or on country lanes where it would be a total waste of time, I'd rather drivers learned how to actually drive and be patient and so on.
I hear what you're saying, but there is plenty of room in cities for segregated cycling infrastructure. The 'problem' is that it has to be taken at the expense of capacity for motor traffic. It's 100% possible, it just needs the political will to make it happen.
Country roads are a different matter, but one that is basically down to driver behavior. Get people cycling in cities though and it becomes a normal activity. Once the majority of drivers have a child/partner/friend/relative who cycles, their attitude to cyclists on country roads will change.
Back to the helmet debate, I always wear one when mountain biking, always wear one when out on club rides on the road and never wear one when I'm just pootling to the shops. I'd stop using my bike for riding to the shops/pub if helmets were compulsory though. The whole point is that I can just grab my bike in my everyday clothes and go.