The thought strikes me that a couple things happen together:
1. Municipalities work to market cycling. They build infrastructure, maybe run billboards ads, put up signs, have educational events, etc to promote people biking.
2. More people bike
3. Safety appears to get better.
(I'm ignoring the kid thing Robert, because it's not my point).
Could it be that the #1 spawns #3 as much, if not more, than #2?