The roman system of justice is considered a milestone because it

Served as a model for later legal systems.

Explanation:

The Roman system of justice is considered an important milestone in the development of law, particularly in the Western world. Roman law was extremely complex and organized, as well as codified. Because of this, Roman law was considered to be an example to the world in later years. Many modern systems of law were directly derived from such a law, while many others have been indirectly influenced by it.

They were being considered as a milestone because it means that the milsetone was being placed as a marker placed along side of the road to tell u the distance is to the nearest city.