To determine if a road is **insufficiently lit**, the visibility of certain elements on the road must be considered. A road is considered poorly lit when **the license plate of the vehicle in front cannot be read at a distance of 10 meters**. This is a clear measure to assess the lighting of the road, as if the license plate cannot be distinguished at that distance, the lighting is insufficient for safe driving.

Additionally, it is mentioned that a road is also poorly lit if a dark-colored vehicle cannot be seen at a distance of 50 meters. However, for the specific question about the license plate, the reference is visibility at 10 meters.

Therefore, if you cannot read the license plate of the vehicle in front at 10 meters, it is a clear indication that the road is not adequately lit.