I don't know if DC is requiring helmet use for adults riding the bikes. That being said, I've been in an accident in which my helmet was cracked in half (head was fine), so I can attest the benefit of a helmet. I never leave home without one affixed to my head. In DC this is of particular interest since the majority of people who come into the city each day are not DC residents. So, if they get hurt in the District and have to be transported to a hospital, it's on the DC taxpayer's dime and not theirs. If they don't have health insurance, it's a double-whammy. From a health care perspective, something as easy as wearing a helmet should be done to avoid costly brain injuries.

That all being said, I agree that there are probably more important things to be worrying about. More people die from obesity than head injuries, so it would behoove cities and states to pay more attention to reducing obesity than enforcing mandatory helmet laws. In addition, in DC, the biggest threat to cyclists is the behavior of automobile drivers. IMO, enforcement efforts should be directed at those people who cause the most damage or have the potential to do so. You're probably far more likely to be struck and critically injured by a motorist in DC than you are to fall off your bike and sustain a head injury.