Concept
The road hierarchy categorizes roads according to their functions and capacities. While sources differ on the exact nomenclature, the basic hierarchy comprises freeways, arterials, collectors, and local roads. Generally, the functional hierarchy can more or less correspond to the hierarchy of roads by their owner or administrator. The related concept of access management aims to provide access to land development, while ensuring traffic flows freely and safely on surrounding roads. At the top of the hierarchy in terms of traffic flow and speed are controlled-access highways; their defining characteristic is the control of access to and from the road, meaning that the road cannot be directly accessed from properties or other roads, but only from specific connector roads. This indirection, in conjunction with high speed limits and multiple lanes, allows these roads to support fast traffic flow with high volume, in both urban and rural areas. They are at the bottom of the hierarchy in terms of access to property, allowing access to almost nothing besides other roads and rest areas. They are also known as freeways in the U.S., South Africa, and parts of Australia; as motorways in the U.K., New Zealand, and parts of Australia; and as expressways in numerous countries. Controlled-access highways do not have traffic signals nor at-grade crossings with other roads (intersections), railways, or pedestrian paths. When a road's path would intersect a controlled-access highway, a flyover (US: overpass or underpass) will vertically separate the two roads, preventing the need for an intersection. Pedestrian footbridges and tunnels are also grade-separated. The opposing directions of travel are separated by a central reservation (UK) or median strip (US) generally a few meters to a few dozen meters wide, which may consist of a traffic barrier or a natural barrier like grass. This separation makes controlled-access highways a subset of dual carriageways (UK) or divided highways (US). Generally, these roads prohibit pedestrians and vehicles not powered by a motor (e.
