Final answer:
Headlights are required during the night and any time visibility is significantly reduced such as in heavy rain, fog, or when visibility is less than 500 feet. Their use increases safety by improving visibility and allowing other motorists to see your vehicle.
Step-by-step explanation:
Headlights are required not only during the night but also in any conditions where visibility is significantly reduced. This typically includes when it is raining, foggy, or when you cannot see persons or vehicles for 500 feet. Modern vehicles often come equipped with automatic headlights that will turn on in these conditions to ensure safety.
As per traffic laws, one of the primary times headlights are required is during the night. However, most states and countries have specific regulations that also require headlights to be used during poor visibility conditions such as heavy rain, fog, or even during heavy dust. Furthermore, many places have a rule that states headlights must be turned on half an hour after sunset to half an hour before sunrise, or at any time when there is insufficient light to clearly see a person or an object at a distance of 500 feet.
The usage of headlights is not only for your own visibility but also to ensure that other drivers can see you, which is why it is not strictly tied to the level of traffic. Therefore, option 3, 'At any time you cannot see persons or vehicles for 500 feet,' is the answer that encompasses most of the scenarios where headlights would be required by law.