We are easily manipulated. A news story with lots of blood convinces us that a minor issue is the problem of the day. For example, the Camera front page has a story about an accident at a flashing crosswalk, and suddenly flashing crosswalks are death traps. We conveniently ignore the fact that most traffic accidents do not happen at these crosswalks.
Then you throw in peer manipulation. One person says that pedestrians don't look before crossing the street. Pretty soon it is common knowledge that pedestrians in Boulder have a death wish and always walk out directly in front of cars without even a glance, an opinion that, if examined logically, couldn't possibly be true.