Suppose a city experiences a sudden and unusual increase in the number of traffic accidents in a particular area. In response, the policymakers reduce the speed limit in the area. Over time, the number of accidents in that area decreases, and policymakers attribute this to the success of their safety measures. Explain why the policymakers might be mistaken in their assessment of the effectiveness of speed limits. (Hint: a good answer would name the phenomenon that biases policymakers' beliefs and explain briefly how it works.)