[1] reports on Australian introduction of mandatory cycle helmet laws, and says:
"Pre-law surveys counted 6072 child cyclists in NSW, 3121 cyclists (all ages) in Victoria; and over 200 000 cyclist movements on two key routes in Western Australia. Equivalent counts a year after enforced helmet laws showed declines of 36% (NSW), 36% (Victoria) and 20% (Western Australia). Sunday recreational cycling in Western Australia (24 932 cyclists pre-law) dropped by 38%. Increases in numbers wearing helmets, 1019 (NSW) and 297 (Victoria) were substantially less than declines in numbers counted (2215 and 1110)."
In other words, Victoria started with 3121 cyclists, gained 297 helmets and lost 1110 cyclists.
Are you sure you didn't misread that 4% figure from a source that actually said 40%?
Needless to say, a 20% reduction in head injuries from a 40% reduction in cycling doesn't seem like a very good deal to me.
"Pre-law surveys counted 6072 child cyclists in NSW, 3121 cyclists (all ages) in Victoria; and over 200 000 cyclist movements on two key routes in Western Australia. Equivalent counts a year after enforced helmet laws showed declines of 36% (NSW), 36% (Victoria) and 20% (Western Australia). Sunday recreational cycling in Western Australia (24 932 cyclists pre-law) dropped by 38%. Increases in numbers wearing helmets, 1019 (NSW) and 297 (Victoria) were substantially less than declines in numbers counted (2215 and 1110)."
In other words, Victoria started with 3121 cyclists, gained 297 helmets and lost 1110 cyclists.
Are you sure you didn't misread that 4% figure from a source that actually said 40%?
Needless to say, a 20% reduction in head injuries from a 40% reduction in cycling doesn't seem like a very good deal to me.
[1] https://injuryprevention.bmj.com/content/9/4/380