In The Netherlands. Nobody wears a helmet, with a few exception for very young kids (Always flanked and shielded by a overly concerned parent.)
I could show a graph that nicely shows that helmets are correlated with higher death rates. (No the helmet doesn't kill, its because helmets are worn in countries with low separation of slow cyclists and fast cars)
There is also a correlation between more helmets (by law) leading to LESS cyclers. Its a burden.
Seperation of slow and fast traffic is BY FAR the biggest factor here. Then also consider the health benefit of the exercise.
Regular exercise will make you more healthy and prolong your life! So, on bike lanes, Cycling is Super Awesome Safe! No helmet needed.
ps, incidents are on the rise due to old folks going faster on their electric assisted bikes.
ps2 mopeds, scooters, especially those that clock 50km/h are more and more forced into the car lane in The Netherlands, the speeds fits better.
ps3 Watch your juveniles, those pesky 12-18 y/o have a high incident rate. They are also likely to be offended by a helmet..