Which of these statements about marking a stopped vehicle is true?

The correct statement emphasizes the importance of visibility for marking a stopped vehicle, especially in situations where obstacles like hills or curves may obscure it from the view of approaching drivers. By moving the reflective triangles back to a distance where they can be clearly seen—typically within 500 feet—drivers have adequate warning to react properly and safely. This is a critical safety measure that helps prevent accidents by giving adequate notice that a vehicle is stopped ahead.

The other statements do not align with safe practices for marking a stopped vehicle. For instance, using reflective triangles is highly recommended, even at night, as they enhance visibility in low light conditions. Additionally, reflective triangles are not optional; they are a standard safety requirement for commercial vehicles. Lastly, while placing triangles on the roof may seem effective, it's not practical as these devices are intended to be placed on the ground at strategic distances to alert oncoming traffic effectively.