What is a required action when any truck, bus, or trailer is parked on a roadway?

Activating hazard warning lights is essential when a truck, bus, or trailer is parked on a roadway because it enhances visibility and alerts other drivers to the presence of the parked vehicle. This action is a key safety measure, as it helps prevent accidents by ensuring that approaching vehicles can see the stationary vehicle from a distance, especially in low visibility conditions like dusk or poor weather.

Hazard lights serve as a warning signal to other motorists, making it clear that caution is required when approaching the parked vehicle. This is particularly important on roadways where parked vehicles might cause an obstruction or unexpected changes in traffic flow. The other options do not directly address the immediate safety needs associated with a vehicle's presence on a roadway.