When should hazard warning lights be activated by trucks or buses?

Hazard warning lights should be activated by trucks or buses when they are parked on a roadway or shoulder as a safety measure to alert other drivers to the presence of the vehicle. This is crucial in situations where visibility may be compromised or where the vehicle could be at risk of being struck by passing traffic. Activating hazard lights helps to signal to other road users that the vehicle is stationary and potentially poses a hazard, thus enhancing safety for both the stationary vehicle and other motorists.

In contrast, the other scenarios do not generally warrant the use of hazard lights in accordance with traffic safety regulations. For instance, parking on a sidewalk is typically illegal and not something that would justify activating hazard lights. While it may be important for safety during rain, merely being in rainy conditions doesn't meet the threshold for activating hazard lights. Lastly, using hazard lights while a passenger is entering or exiting the vehicle is not standard practice unless the situation involves an emergency or unusual hazards.