Outside has an interesting look at unintended consequences, examining a University of Colorado study into why bike-helmet laws increase injuries:
Studies show that the laws deter people from riding, and the fewer riders there are on roads, the more car-bike accidents occur. Research backs up the idea that more cyclists on city streets reduces the number of collisions with automobiles, though exactly why is unclear. Anti-helmet-law groups believe it’s because motorists become accustomed to sharing the road. “Nothing makes cycling safer than more cyclists,” says Keegan Stephan, an organizer for New York City biking advocacy group Right of Way. “It changes the way drivers behave, and it changes the culture on the streets. Passing helmet laws that will deter cycling is one of the more detrimental things you can do.”