What is the primary purpose of official traffic-control devices?

The primary purpose of official traffic-control devices is to regulate, warn, and guide traffic. These devices include traffic signs, signals, and road markings, each designed with specific functions to ensure the safety and efficiency of roadways.

Regulating traffic involves enforcing laws and rules that drivers must follow to maintain order on the roads, such as stop signs and speed limits. Warning devices, including caution signs about potential hazards like curves or pedestrian crossings, help alert drivers to conditions that may require them to adjust their speed or be more vigilant. Additionally, guiding traffic means directing drivers on where to go and how to navigate the roadways effectively, as seen with directional signs and lane markings.

In contrast, the other choices do not align with the fundamental function of traffic-control devices. While landmarks and advertisements may serve various purposes, they do not contribute to the regulation or safety of traffic in the same way as official devices do. There is no intent for entertainment in traffic signals or signs; they serve a critical safety function. Thus, the correct answer highlights the essential role these devices play in promoting safe and efficient movement on the roads.