When is it illegal to use high beam headlights?

Using high beam headlights is illegal when there are oncoming vehicles because high beams can significantly impair the vision of other drivers. The bright light from high beams can create glare, making it difficult for oncoming drivers to see the road and react appropriately. Therefore, to ensure safety for all road users, it's essential to switch to low beams when encountering oncoming traffic.

In contrast, using high beams in conditions like fog can also be problematic, as the bright lights can reflect off the moisture in the air, causing further visibility issues. In urban areas, while it’s generally better to use low beams due to street lighting and the presence of pedestrians, it is the presence of oncoming vehicles that is the strict rule regarding high beam usage. At night, particularly when no other vehicles are present, high beams may be used as they provide better visibility.