Depressed roadway

A controlled-access highway is a type of highway which has been designed for high-speed vehicular traffic, with all traffic flow and ingress/egress regulated. Common English terms are freeway (in Australia, South Africa and parts of the United States and Canada), motorway (in the United Kingdom, Ireland and New Zealand) and expressway (some parts of Canada, parts of the United States, and many Asian countries). Other similar terms include Interstate and parkway. Some of these may be limited-access highways, although this term can also refer to a class of highway with somewhat less isolation from other traffic.

In countries following the Vienna convention, the motorway qualification implies they are forbidden for walking or parking, and reserved for the use of motorised vehicles only.

A controlled-access highway provides an unhindered flow of traffic, with no traffic signals, intersections or property access. They are free of any at-grade crossings with other roads, railways, or pedestrian paths, which are instead carried by overpasses and underpasses. Entrances and exits to the highway are provided at interchanges by slip roads (ramps), which allow for speed changes between the highway and arterials and collector roads. On the controlled-access highway, opposing directions of travel are generally separated by a median strip or central reservation containing a traffic barrier or grass. Elimination of conflicts with other directions of traffic dramatically improves safety and capacity.

Controlled-access highways evolved during the first half of the 20th century. Italy opened its first autostrada in 1924, A8, connecting Milan to Varese. Germany began to build its first controlled-access autobahn without speed limits (30-kilometre (19 mi) on what is now A555, then referred to as a dual highway) in 1932 between Cologne and Bonn. It then rapidly constructed a nationwide system of such roads. The first North American freeways (known as parkways) opened in the New York City area in the 1920s. Britain, heavily influenced by the railways, did not build its first motorway, the Preston By-pass (M6), until 1958.

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