Which of the following is most important for increasing visibility during heavy rain?

Turning on headlights is crucial for increasing visibility during heavy rain. When it rains heavily, the amount of light reflected off the water on the road surface, along with reduced visibility due to the rain itself, can make it difficult for drivers to see both the road and other vehicles. Activating headlights helps to illuminate the road ahead and makes the vehicle more visible to others, reducing the risk of accidents.

Additionally, many states have laws requiring headlights to be on during inclement weather to improve safety for all road users. This practice enhances your ability to see further ahead while also ensuring that other drivers can see you, which is especially critical in heavy rain conditions.

While other options like using fog lights or ensuring a clean windshield contribute to visibility, turning on headlights is the most effective and standard approach for improving visibility in heavy rain.