Atlantic Cities (read a few of the comments). Info about helmets at Bicycling Magazine.
Before this argument even picks up a faint hint of steam it needs to be crushed. Mandatory bike helmet laws would be a foolish step to take if we are at all serious about lowering carbon emissions. If you are so concerned about the safety of cyclists then lobby for tougher laws against aggressive drivers and let individual cyclists decide whether or not they want to wear a helmet.
Making helmets mandatory for cyclists is basically telling people not to ride bikes. It over-inflates the dangers of cycling and gives people the false impression that riding a bike is hazardous to their health. I have ridden a bike almost every day of my life since I was a child and the vast majority of the kilometers I have racked up cycling have been without a helmet, so sue me. If you look at cities where people actually embrace cycling as a mode of transportation you will find that almost no one wears a helmet when commuting. You can cite all of the studies you like (and the argument about helmets goes both ways—for and against) but I haven’t completely relinquished my common sense which tells me that riding a bike in the city isn’t particularly dangerous.
I’m quite certain that studies would show that people who wear helmets 24 hours a day no matter what they are doing suffer fewer head injuries yet we don’t mandate helmet laws for all public activities. Wearing a helmet while driving a car would probably save lives but making all automobile passengers wear a helmet would be at least as ridiculous as forcing helmets onto the heads of bike commuters.
The bottom line is that society should be getting down on their hands and knees and thanking people who choose to ride a bike instead of firing up an internally combusted machine to effect their daily rounds. The absolute last thing we need to do is give anyone an excuse to leave their bike at home.