What is the appropriate action for a truck that is stopped on an adjacent shoulder?

The appropriate action for a truck that is stopped on an adjacent shoulder is to actuate electric hazard warning signal lights. This action is crucial as it serves to alert other drivers on the road that there is a vehicle temporarily parked in a potentially hazardous situation. By using hazard lights, the driver increases visibility and reduces the risk of a collision.

Other options, although they may seem reasonable in certain contexts, do not prioritize the immediate need for safety on the road. Turning off the hazard lights would not be advisable when the vehicle is in a stopped position, as it would eliminate the warning signal to other motorists. Calling for a tow truck, while necessary to resolve the situation eventually, does not address the urgent need for safety until assistance arrives. Waiting for medical help may be relevant if there are injuries involved, but it still does not serve to protect the vehicle from potential accidents in the meantime. Therefore, actuating the hazard lights is the best course of action to ensure safety and alert others.