When driving through a **sufficiently illuminated underpass**, it is necessary to turn on both the **position lights** and the **dipped beam headlights**.

- **Position lights**: These are used so that other drivers can see your vehicle, especially in situations where visibility may be limited, such as in tunnels or underpasses. Even if the underpass is well-lit, these lights help make your vehicle visible to others.

- **Dipped beam headlights**: These are mandatory during the day when passing through tunnels or underpasses, regardless of the lighting. These lights improve your visibility and that of other drivers, ensuring safe driving in conditions where natural light is not sufficient.

Therefore, even if the underpass is well-lit, the regulations require the use of both types of lighting to ensure road safety.