When should a driver use low beams when driving at night?

Using low beams when driving at night is essential for both safety and visibility. Low beams are specifically designed to reduce the glare that can occur for oncoming drivers and to enhance the driver's ability to see the road without causing distraction. The correct time to use low beams is during conditions such as fog, rain, or when another vehicle is approaching.

In foggy or rainy conditions, low beams help to illuminate the road without reflecting off moisture in the air, which can happen with high beams. This is vital for maintaining visibility and ensuring that you can see road markings, pedestrians, or other obstacles clearly. Additionally, using low beams when another vehicle is approaching prevents temporary blindness from glare, which enhances safety for all drivers on the road.

Other scenarios described, such as complete darkness or driving speeds under 20 mph, do not fully capture the practical application of low beams. High beams are generally used in dark areas without oncoming traffic, while driving speed does not solely dictate the need for low beams. Therefore, option B comprehensively encompasses the right circumstances for using low beams effectively at night.