Roads, defined as a path connecting two points, are generally classified into a hierarchy. At the top of the hierarchy are freeways (US) (UK: motorways), which serve entirely a function of moving vehicles between other roads. Freeways are grade-separated and limited access, have high speeds and carry heavy flows. Below freeways are arterials. These may not be grade-separated, and while access is still generally limited, it is not limited to the same extent as freeways, particularly on older roads. These serve both a movement and an access function. Next are collector/distributor roads. These serve more of an access function, allowing vehicles to access the network from origins and destinations, as well as connecting with smaller, local roads, that have only an access function, and are not intended for the movement of vehicles with neither a local origin nor destination. Local roads are designed to be low speed and carry relatively little traffic.
The design of roads is specified in a number of design manual, including the AASHTO Policy on the Geometric Design of Streets and Highways (or Green Book). Relevant concerns include the alignment of the road, its horizontal and vertical curvature, its super-elevation or banking around curves, its thickness and pavement material, its cross-slope, and its width.
Fundamentals of Transportation/Geography and Networks
In English-speaking countries the term is used in the United Kingdom (a motorway is also called a traffordd (plural: traffyrdd) in Welsh), parts of Australia, New Zealand, Pakistan, some other Commonwealth nations, and Ireland (a motorway is also called a mótarbhealach (plural: mótarbhealaí) in Irish). In Ireland, a road built to motorway standard, but without the designation (and the regulations and traffic restrictions resulting from that designation), is known as a High-quality dual carriageway.
View over the cirque of Pougnadoire and Saint-Chély-du-Tarn village, in the Tarn Gorges, from the cirque of Saint-Chély. This picture features the road D 907bis which goes alongside the Tarn river all along the gorges. Credit: (Benh)
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