What type of signal is required by law for trucks, buses, and trailers parked on roadways under specific conditions?

The law requires that trucks, buses, and trailers parked on roadways under specific conditions must display warning signal lights. These lights serve as a crucial safety feature, alerting other drivers to the presence of a stationary vehicle, especially in conditions of low visibility or during nighttime. The use of warning signal lights is intended to reduce the risk of accidents by ensuring that approaching vehicles can see and respond to the potential hazard created by the parked vehicle.

While other options, such as signal flags and flashing headlights, can provide visibility, they are not specifically mandated for parked vehicles on roadways. Reflective triangles can be used as an additional safety measure; however, their primary function is typically as a roadside emergency device rather than a legal requirement for parked vehicles. Therefore, the obligation to employ warning signal lights helps ensure compliance with safety regulations aimed at protecting both the occupants of the stationary vehicles and the other roadway users.