Which of the following is NOT a safety feature in the Highway Transportation System?

In the context of the Highway Transportation System, safety features are designed to enhance the overall safety and efficiency of roadways for vehicles and pedestrians alike. Guard rails, electronic notification signs, and street lights all serve specific safety functions.

Guard rails are installed to prevent vehicles from leaving the roadway and to reduce the severity of crashes. Electronic notification signs provide real-time information to drivers about traffic conditions, hazards, or other important messages, promoting safer driving decisions. Street lights illuminate roadways at night, enhancing visibility and making it safer for all road users.

Heavy traffic signals, while they do control traffic flow at intersections and help prevent accidents, are generally considered more of a traffic control device than a safety feature like the others mentioned. The term "heavy traffic signals" implies a focus on managing traffic volumes rather than directly enhancing safety through physical barriers or warning systems. As such, they do not fit within the broader category of safety features in the Highway Transportation System in the same way that the other options do.