First things first, I mean no offence to anybody.
I would have loved to believe this rule will be effective (especially in Delhi with little security for women, untoward incidents await each day) but women just don't seem to understand the importance of helmet or anything else for that matter (e.g. riding sanely, seat belts). Just because they know the mob justice will be on their side they can do whatever they want. C'mon, I've seen you girls ride and drive so dangerously. I witnessed an event some days back (well, not me but my mom did) : a guy was riding sanely and a girl made an idiotic move. Both the riders fall down, and there is no damage done to any vehicle or rider. Yet the girls insults the guy with the choicest words. Poor guy just listens and moves on.
So it's more of women's mentality that may result in this rule being a failure or success. I do not think anything will happen unless women understand the importance of helmets. They just won't listen to their husbands or boyfriends or even parents (Or anybody who has been saved because of helmets) I reckon.
Again, no offence meant and an individual is always unique, so this might not apply to the girl reading this.