To understand why it is mandatory to turn on the **high beam headlights** when driving at more than 40 kilometers per hour on a **poorly lit interurban road**, it is important to know the rules regarding the use of these lights.

- The **high beam headlights** must be used on **roads outside towns or cities** when there is little natural light, especially at night. This is crucial to improve visibility and safety when driving at speeds over 40 km/h.

- A road is considered **poorly lit** if you cannot read the license plate of the vehicle in front at 10 meters or if you cannot see a dark vehicle at 50 meters. In these conditions, high beam headlights are necessary to ensure that the driver can see properly and be seen by others.

- In contrast, on **sufficiently lit roads**, it is not necessary to use high beam headlights, as the existing lighting is adequate for safe driving.

Therefore, on a **poorly lit interurban road**, it is mandatory to turn on the high beam headlights to ensure adequate visibility and comply with road safety regulations.