What is the primary function of daylight running lights on vehicles?

The primary function of daylight running lights is to enhance visibility for other drivers. These lights are designed to make vehicles more noticeable in daylight conditions, which can significantly improve safety by reducing the chances of collisions. When vehicles are equipped with these lights, they emit a consistent light that makes the vehicle easier to see, even in bright sun or during overcast conditions. This increased visibility helps other drivers and pedestrians to better gauge the distance and speed of the approaching vehicle, ultimately leading to safer roadways.

In contrast, other options like improving fuel efficiency or serving merely as a decorative feature do not align with the safety purpose that daylight running lights serve. Reducing glare for oncoming traffic is not their primary function, as these lights are intended to enhance visibility rather than limit glare.