What is required to be displayed when vehicles or machinery are parked on a highway or adjacent shoulder?

When vehicles or machinery are parked on a highway or adjacent shoulder, it is essential to display red flags to enhance visibility and alert other roadway users of potential hazards. Red flags are recognized as a standard safety measure that indicates the presence of work zones or obstruction on the road, especially in situations where vehicles may not be clearly visible to oncoming traffic. This practice helps in preventing accidents and ensures a safer environment for both the operators of the parked equipment and other motorists.

While other options may contribute to safety in different contexts, red flags specifically serve the purpose of signaling the presence of parked vehicles or machinery on the highway, making them the correct choice in this scenario.