**Special vehicles** that perform work on roads, such as construction or maintenance services, must use a **yellow light** to alert other drivers of their presence. This light is mandatory on both **motorways** and **dual carriageways** from the moment they enter until they reach their destination. The purpose of this light is to improve the visibility of the special vehicle and ensure safety on the road.

The **V-2 signal**, which is a yellow light, must be on **both during the day and at night**. This ensures that the vehicle is visible at all times, regardless of natural light conditions. The regulations do not differentiate between daytime or nighttime use, which means the signal must be active whenever the vehicle is operating on the road.

Therefore, the correct answer is that drivers of special vehicles must use the V-2 signal **both during the day and at night** to comply with road safety regulations.