Bike Helmets

When, exactly, did everyone stop wearing bike helmets?  It seemed like everyone was wearing them for a long, long time and then, all of the sudden, I’m seeing way more people on bicycles sans helmets.  Did I miss some sort of memo?  I mean, just the other day I was out for a run and saw a women with a toddler in a babyseat on her bike and both the woman AND THE TODDLER didn’t have helmets on.  Who does that?