In order to apply advanced high-level concepts for transportation networks, like hypergraphs, multi-level wayfinding and traffic forecasting, to commercially available street network datasets, it is often necessary to generalise from network primitives. However, the appropriate method of generalisation strongly depends on the complex street network feature they belong to. In this paper, we develop formal expressions for road segments and some essential types of roads, like roundabouts, dual carriageways and freeways. For this purpose, a formal network language is developed, which allows a clear distinction among the geometrical network, its embedding into the Euclidian plane, as well as navigational constraints for a traffic mode.