New Zealand made bike helmets required in 1994. Cycling data from 1988-91 and 2003-07 shows that average hours biked per person in NZ declined 51% and overall injury rates per hour biked increased 20%. There was a 75% reduction in biking for children 5-17 years old.
Decline in hours cycled data
Injury data
Source: Evaluation of New Zealand’s bicycle helmet law, 2012