Studies have shown which of the following to be a serious traffic hazard?

Answer: C — Slow drivers

The correct answer identifies slow drivers as a serious traffic hazard. Slow drivers can create dangerous situations on the road because they can disrupt the flow of traffic. When a vehicle is traveling below the normal speed limit, it can cause frustration and impatience among other drivers, which may lead to risky maneuvers such as overtaking in unsafe conditions. This can increase the likelihood of accidents, especially if other drivers are not anticipating a slower vehicle in a faster-moving lane. In contrast, while middle-aged drivers may have certain characteristics that could affect their driving, they are not inherently considered a serious traffic hazard based solely on their age. Traffic hazards tend to be more associated with actual behaviors or conditions, such as driving significantly slower than the average speed, rather than the demographic characteristics of drivers.