Final answer:
You should turn on your vehicle's headlights when you cannot clearly see persons or vehicles at a distance of about 500 feet (150 meters), to ensure road safety for yourself and others.
Step-by-step explanation:
In most regions, you must turn on your vehicle's headlights when visibility is such that you cannot see other persons or vehicles at a distance of typically about 500 feet (150 meters). This visibility guideline ensures that you have enough time to react to potential hazards on the road. Headlights are not only for your ability to see others but also so that other road users can see you, which is why it is crucial to have them on in low visibility conditions.
The specifics of when you must have your headlights on can vary by jurisdiction, but the general principle is to enable them during any conditions that significantly reduce visibility, such as at dusk, dawn, during fog, heavy rain, or snow. Additionally, some areas have laws requiring the use of headlights from a specified half-hour after sunset to a half-hour before sunrise, or any time when windshield wipers are necessary as a result of precipitation or mist.