**Special vehicles** that travel on public roads must use a **yellow warning light** both during the day and at night. This light is important because it alerts other drivers to the presence of these vehicles on the road, especially when they are performing improvement or maintenance work. Although they do not have priority to pass or overtake, the yellow light helps them to be visible and maintain safety on the road. Therefore, it is necessary for the light to be on at all times, regardless of whether it is day or night, to ensure that other drivers are aware of their presence.