The sign "Length of a distance of ... km" indicates the length of a road or section where a specific condition applies, such as a speed limit, restriction, or other regulations.

In this case, the sign typically shows a number followed by "km," indicating how long the condition will be in effect starting from the point where the sign is placed. For instance, if the sign indicates "Length of a distance of 3 km," it means that the driver should adhere to the given road rules for the next three kilometers.

It's essential to pay attention to these signs as they inform drivers of how long they need to comply with particular regulations, which helps ensure safety on the roads. If you have any more specific questions or need further clarification, feel free to ask!